not a clover but no idea what it actually is!

Thanks.

I have seen this before.. I think it is a weed called......
Medic Black
An annual or short lived perennial legume with trailing stems that grow close to the ground. The taproot penetrates deeply into most soils. The three-leaflet leaves have prominent veins and are similar to most other clover leaves. The small clusters of flowers are bright yellow. Seed pods turn almost black at maturity. Black medic is most noticeable in lawns during June, July, and August.

Thank you Lady Essex, I will be rooting out as much as I can before it over runs the garden.

I'm not convinced its Black Medick for two reasons. That is usually prostrate and this is a clump. And the leaves of the medick are very slightly indented at the tip and there is a tiny hair like little spike at in the indent and neither of these features shows in the photo. But if its coming through from next door it would probably be wise to take it out anyway.

It looks like a type of Trifolium.

Maybe Trifolium dubium? See webpage:
http://meltonwiggins.com/smallhopclover.html

That looks more like it - see if it has little yellow flowers.