Oxalis corniculata, creeping woodsorrel, it's very hard to eradicate. It has explosive seed capsules, and produces bulblets on the roots, so whenever you try to pull it up you're propagating it instead...
Glyphosate works tolerably well, but it'll keep coming up so you have to keep spraying, which not everybody would want to do.
But you do need to get it under control if you can.
Sorry it wasn't better news.

You may look on it a little more kindly when you know one of its popular names is Sleeping Beauty ...but it spends more time spreading than sleeping.

Thank you Teadrinker for your helpful response, albeit a little depressing! I'll try Glyphosate as you suggest.

Thank you Steragram for your reply. I suppose the thing to do is to try and catch it whilst it's sleeping!

if its in gravel regular hoeing/raking will help but I'd also go for the weed killer option.

Lol Edukes, never thought of that!