What to do if your bike is stolen

Yesterday my friend managed to retrieve a stolen bike that a fellow lfgss member spotted on gumtree. The following advice is based on what worked for us (and is mainly a repost from the stolen bikes thread):

If you see your stolen bike for sale online this is what to do:

Arrange to meet the seller by text message in a busy public place, during the week, and in daylight hours. (eg; a tube station).
Then contact the police station nearest to where you have arranged to meet the seller, and explain the situation.

The police don't have the authority to pose as buyers, so in arranging the meeting yourself, you will be bypassing a lot of red tape, and allowing the police to act far faster than they would otherwise be able to.

DO NOT go to meet the seller yourself; it is more than likely not worth the risk. The police will take care of everything once they know the meeting place / time.

They go in, pose as buyers and make the arrest, whilst you stick around to give statements / get your bike back.

So;
ARRANGE THE MEETING YOURSELF VIA TEXT MESSAGES.
THEN CALL THE LOCAL POLICE AND LET THEM DEAL WITH IT.

I have been looking for a bike secondhand on Gumtree and Ebay. Been doing quite well so far in finding sellers who have all the requisite info to prove it's theirs and they came by it honestly. Unfortunately for me the frame sizes always turn out too big.

I've just rung the guy about a ridgeback velocity advertised on gumtree. It's got a stock photograph. info in the title includes that it's 15" is a ladies frame and gives the retail price and it's age.

and I'm pretty sure it's a stolen bike...

I asked him if he had proof of purchase - "yes yes of course"
I said can I ask the reason for selling - "i don't think you are a real buyer" and hung up immediately!

What can I do?
It's not my bike which I'm suspecting is stolen, and I'm not in a fit state to go and meet him.
Any ideas, I'm pretty sure from his responses and his general shifty demeanor that he's moving someone's stolen bike on.