Stolen bikes

I suspect the teenagers downstairs from me are fences for stolen bikes, or even nicking them themselves. Every few days their 'bikes' change. I find it hard to believe as they seem really nice polite lads.

I'm starting to take pictures of every new bike I see, should I be involving the police at this stage or should I do further stake out? I'm nervous about bringing in the law in case they find out it was me that dobbed them in. As much as I fucking hate bike thieves, I also value my own property and am wary of possible retaliation.

Advice welcomed ...

where
we could arrange a stake out get details of bikes and see if they really are stolen