Wilko have several types of battery operated Xmas lights, Coloured or plain white, bought a set last week for a tree we have outside (they work great even in the bad weather we have had), I wanted to get another set, but they were sold out on line and I couldn't find any in my local Wilko (Wellington), but popped in today and they have them in stock again, 50 lights for £4 (Half price) in white and the coloured ones, they come with 3 batteries and switch on for about 6 hours a day on the timer, the first set I have has been working for a week on the same batteries. (Sorry for rambling on, below is the blurb from the website).

Let there be light! This energy saving set of 50 multifunction white lights or 4 colours has an added timer function, so they'll switch on automatically each day. These versatile battery operated lights are suitable for both indoor and outdoor use and come with 9 functions and a discreet black cable.

Total length approximately 4.7m. Distance between lights 8.5cm. When the optional timer is activated, the lights will automatically turn on each day for approximately 6 hours, and turn off every day for approximately 18 hours. Requires x3 AA batteries (included).

Although these bulbs are non-replaceable, they are highly energy efficient and have a lifespan far greater than that of ordinary filament bulbs.

Warning:
Keep away from direct heat or flame.