Have you looked on Nextdoor? I saw a post there for a lost gold necklace in clapham but I don't think it fits your description. But someone else on there may have seen it. 

I lost a gold pendant around Northcote Road, on Saturday 22nd March. It is a rectangle shaped, with an angel in the centre. The name Araceli is engraved on the back. It holds huge sentimental value, and is a family heirloom. We are offering a reward to anyone who helps find it- we would be so grateful!
Please contact hannah16cj@gmail.com