If it was stolen from outside the Tate Britain, I could have told you with some level of certainty where that had ended up, as a few bikes from outside my work have ended up being stolen by the same people, but sadly I don't know much about the Tate Modern crime syndicates.
I'll keep my eye out for your bike though - gutted for you and really brings home how much personal value/sentiment there is in a bike, whether you owned it from new, or it is new to you.
Do you have any pics of it that will help identify it?