Considering what DIY outlets charge for cable, you might be better off finding an extension cable reel that's already made up - especially if it's on special offer. Unless you have a cheap source for your cable, plug, and socket, that is.
25 meters is a long run and will have a great deal of resistance. although 1.5 mm cable would satisfy a 13 amp plug top under normal circumstances. for that lenght of run, i would use 2.5 mm core flex.