According to the T&Cs, the promotion officially begins when we first see the offer on Nectar, if no start date is mentioned:

Bonus offer terms and conditions

Promotion Period

This promotion ("Promotion") will be open from the start date as stated on the banner/e-mail/other communication regarding this Promotion ("Marketing Materials") or, if no such date is stated, from when you first saw the Marketing Materials until the end date as stated on the Marketing Materials ("end date"). This period will be known as the "Promotion Period".

Alternatively, the Promotion Period may be limited to the number of times the Promotion offer can be used. If this is the case, it will be clearly stated in the Marketing Materials.

Eligibility

Only eBay.co.uk registered private users who have been invited to participate in the Promotion are eligible. You must be a UK resident to register for a Nectar card and take part in the Nectar programme. Please find below the eligibility requirements for the various types of bonus offers, unless otherwise stated in communications from eBay.

When you purchase an item: You must be registered on http://www.eBay.co.uk (the "Site") and have a Nectar card linked to your Site account.
When you link your Nectar and Site accounts: You must be registered on the Site and have no existing Nectar account linked to your Site account. To receive the bonus offer you must either link your Nectar account number to your Site account or sign up for a Nectar account on the Site. The bonus offer applies once only the first time you either link your Nectar account number to your Site account or sign up for a Nectar account on the Site. Relinked accounts or multiple applications will not qualify.
When you spend a specific amount on eBay.co.uk during a given time frame: You must be registered on http://www.eBay.co.uk (the “Site”) and have a Nectar card linked to your Site account. You must also spend the required amount on eBay.co.uk before the promotion end date specified on the marketing creative.
Promotion details

The bonus Nectar points that you receive will vary from promotion to promotion. As an example only, the promotion offer could be '100 bonus Nectar points' or 'triple Nectar points'.

Bonus points will be awarded within 28 days of the end date. To see how many bonus points you will receive for the specific item(s) you wish to purchase, please click the 'See details' link on the page where the item is displayed, underneath the box where the item price is displayed.

Alternatively, your bonus points will be displayed in My eBay – Nectar page within 28 days of the end date.

The bonus points displayed should not be assumed to apply to any other item or to anyone else. Please always refer to the page where the item is displayed to see how many points you will receive for your purchase. This amount includes any points gained as a result of this Promotion. Other people may receive the same Promotion.

There is no cash alternative and the offer is not transferable.

Promotion restrictions

There may be restrictions around eligible categories, minimum spend required (basis of minimum spend will be calculated according to the “Collecting Nectar Points” section of the Nectar programme T&Cs) and/or listing type (auction style, fixed price etc.). The Promotion may also be restricted to particular sellers. Any of these or any other restrictions will be stated in the Marketing Materials.

Interaction with other promotions and offers

If you are eligible for multiple Nectar related promotions that have been communicated to you by eBay, then you will automatically gain the benefit of the most valuable one, as determined by eBay.

If you are eligible for any Nectar related promotions that Aimia/Nectar communicated to you, then please refer to any applicable terms and conditions to those promotions for details. You may be able to take advantage of this Promotion as well as any promotion that Aimia/Nectar communicated to you and benefit twice. Please note that the amount of points displayed on the page where the item is shown only takes into account promotions communicated to you by eBay.

Exclusion From Participation

Any attempt to manipulate the Promotion is prohibited and will result in exclusion from participation.

If eBay has suspicions or knowledge that you have been involved in any actual or attempted fraudulent or illegal activity, eBay reserves the right to disqualify you and anyone else involved in such activity. eBay may also disqualify you if you do not act in accordance with these Terms and Conditions, eBay's User Agreement, the Nectar Programme Terms and Conditions, and/or any policies that eBay may communicate to you.

If your Nectar card is/becomes blocked, closed or restricted for any reason during the Promotion Period, you are/will be excluded from participating in the Promotion.

Limitation of liability

eBay makes no representations or warranties of any kind regarding any product or service provided by any third party in connection with any Promotion.

eBay shall not be liable, in contract or tort (including negligence) or the breach of statutory duty or in any other way, for the loss or damage howsoever arising suffered by you resulting directly or indirectly from entry to this Promotion. Nothing in these terms shall limit eBay's liability for death, personal injury or fraud.

eBay will not be responsible for any third party's performance or failure to perform any services related to any Promotion, or for any costs, damages, accident, delay, injury, loss, expense, or inconvenience that may arise in connection with the use of the Promotion, provided that nothing shall limit eBay's liability for death or personal injury caused by its negligence.

eBay does not accept, and is not responsible for, any lost or delayed entries. eBay reminds you of the nature and limitations of the Internet and is not responsible for any problems or technical malfunctions of any communications network, online system or computer hardware or software that may affect entry to the Promotion.

Although eBay will use reasonable efforts to ensure that all information relating to the Promotion ("Promotion Material") is accurate, eBay is not responsible for any problems or technical malfunctions of any communications network, online system or computer hardware or software failure that may affect eBay's operation of the Promotion or the site, including without limitation the accuracy of any Promotion Material.

General

eBay reserves the right to suspend, change or cancel this Promotion at any time, in the event of circumstances arising which in eBay's opinion make it necessary for it to do so. eBay reserves the right to add additional terms and conditions for certain parts of this Promotion. This includes the ability to substitute the offer, which is, in eBay's reasonable opinion, of equivalent value or change the specification of the offer if circumstances require it.

eBay reserves the right to verify your eligibility for this Promotion.

eBay may use third parties to facilitate one or more elements of the Promotion. These third party service providers are subject to confidentiality agreements with us and other legal restrictions that prohibit the use of the information we provide them for any other purpose except to facilitate the specific outsourced eBay related operation. See the Privacy Policy for further information.

These Terms and Conditions are governed by the law of England and Wales. Each party irrevocably agrees to the exclusive jurisdiction of the English courts over any claim or matter arising under these terms and conditions.

Please check out the full terms and conditions relating to the Nectar programme on eBay. Please remember:

If you receive a refund for any reason, the awarded points will be withdrawn.
Certain items or sellers may be ineligible for Nectar points, outside any Promotion related restrictions. eBay reserves the right to rescind points issued in error.
Organiser

The Promotion is organised by eBay Europe S.à.r.l. It may carry out this organisation through its local subsidiaries. Certain elements may be carried out by Aimia Coalition Loyalty UK Limited ("Aimia").

sorry, was my first post and I got a bit excited.

I called Nectar for clarification. The person at the call centre was friendly and polite, but after several minutes of putting the question all I could get was the stock answer: "make a purchase of £18 and points will be credited .....".

Useless.