BoPeep. I don't suppose you could try and get a photo of the cat the next time it visits you - and then post the photo here?
(I know it isn't my B&W cat because he wouldn't venture that far - nor does he go missing! We are a 'fair stretch' further away - in cat terms - even though I know they wander!)
Also, do you belong to Balham Streetlife?
https://www.streetlife.com/area/balham/
It's another good, local forum and there are frequent posts about lost and found cats/pets - so it might be worth your while posting on there too.
I know there's a cat who lives in Hearnville Road who likes 'visiting' - but I haven't seen it for yonks and can't remember its colouring now.
Although maybe the owners of the small shop on Chestnut Grove might be familiar with your visiting cat?!