Its either one of the speedwells or Soleirolia, common name Mind Your Own Business. According to the RHS, the latter is resistant to all lawn herbicide treatments available to gardeners in the UK, but speedwells aren't, so I'd try Verdone Extra, watered or sprayed on according to the instructions on the bottle. If that doesn't work, then its likely Mind Your Own Business, unfortunately, which means digging it out. More info here

https://www.rhs.org.uk/advice/profile?pid=348

Thanks that's really helpful , I have a feeling I will be doing some digging !

I think it is soleirolia, my brother has it in his gravelled area.

Looks like what is proliferating on my gravel. i go with SBG's ID.