2 kids with face covering riding on their bikes, wheeling a third bike by hand. I asked the first man I saw for help.
I convinced him that the bike was stolen, then we followed the kids into the park and shouted "Oi, that is my bike". The kids dropped it and rode away faster than I could blink.
Last night I managed to get a stolen bike off 2 kids in Vicky Park as they were riding away.
Its a basic silver / black road bike from a major brand, probably come from Hackney Wick where I live.
I have handed it to Bethnal Green police station, PM me if you / your friend has lost a bike fits description and I will give you the crime reference.