In reply to Dominicandave:
My bargain xmas gift for kids this year has been MyMonopoly from Sainsburys. I paid a tenner for it but they're down to 6 quid something now (from £20). You get a load of stickers and things you can draw or print on from an inkjet and can make up your own game.
My lad likes monopoly and was asking for Simpsons or Star Wars monopoly but now he's seen this in the shop he's already designed The Big Bang Theory (TV show) Monopoly. That was a few weeks ago and he's nearly forgotten about it now, so he'll be made up on xmas day.
If they're into minecraft, I got a box of cardboard models a few weeks ago, cost a tenner but makes 30 minecraft bits that you need a big box to store it all in. Good value. like this but different items in it:
http://www.argos.co.uk/static/Product/partNumber/1617289.htm
Absolute bargain I got off Ebay, so too late now, was an organic Chemistry modelling set, those little balls and bonds to build molecules, £8 from some chinese seller (there's loads of ads but it's all the same seller). Same thing is £30 most places.