I'm searching for my recently stolen Langster at the moment, so I've read up about this.
You can setup email alerts on gumtree/ebay so make sure you do that, also if your bikes had some nice parts search for them too (if you see something dodgy look at there history for other parts of your bike, in gumtrees case this can be done by googling the mobile number)
Best advice for recovery I can give is to arrange a viewing (in a very public place, and as ever be very careful) then phone you local police station and ask them to come with.
This is because the police need to fill out all sorts of forms to either raid the home or arrange a sting (they may also just be 'too busy')and by you arranging this it skips that step and forces their hand.
I also found an article on someone who did find there bike and their struggle to get it back *link*
P.S If your in London the bikeshd service might help, if not drop me an email (email@example.com) as I've designed a similar service covering the UK that I'm using in my own search