Had a UK seller on ebay do this to me. Won a jet at a crazy low price and then he would not commit to sell. Had to get ebay involved and got my funds back, but really wanted the jet. Good luck with all this.
Hi, there is a Gumtree resolution centre http://help.gumtree.com/articles/General_Information/Reporting-issu... I've never used it though, and don't know how good it is. Unfortunately, I suspect the best it will do is get you your money back rather than the lamp.
Go for it bro, the truth will set you free!
Paypal won't mess around with the seller. You should get your money back pretty quickly.