Unless you are spraying water into the power plugs, (which is just plain dumb), it is not a "must".

I believe "heavy duty" has been addressing the physical strength and not current carrying ability at 10A.

Does not mean that I would not put a 13A fuse in if it if I had to.

If you absolutely must use an extension cable with a pressure washer then an IP65(min) housing is must. See here
Pretty much what Jasee was alluding to (for external electrical items that involve water/moisture) from the outset.
Alternatively get a longer pressure hose perhaps?

Pressure washers

North and South?

I agree it's a poorly worded description "Ideal for outdoor power applications" but then again common sense would/should prevail when using any electrical item whilst it's raining. It's a real pita using my hairdryer in a thunderstorm :wink: Taken as just an extension cable (rather than a permanent outdoor fixture) for £7.99 I think it represents reasonable value.

Does it never rain where you are then? A Rcd alone will prevent current leakage to earth but won't prevent a potentially rather unheathy flow between the poles

great timing. just what I needed!

Damn ordered more expensive one two days ago. Typical.

10A also not very useful

I wouldn't say this ideal for outdoor power applications, it's not waterproof and there should be a rcd (residual current device) somewhere between it and the supply.