[Littlepeahead]

The first thing with any bike through Gumtree is to try and establish if it's stolen, as many are. There's some good advice here.

http://lcc.org.uk/articles/how-to-av...-a-stolen-bike

Use it as a checklist when you contact the seller. If the bike is only a year old you'd expect them to have the receipt. And if they are genuine they should have no objection to showing it to you.