It is Ranunculus ficaria. It can be pretty invasive in damp ground but weed - depends if you like it or not!

Otherwise known as Lesser Celandine. Pretty, but can be hard to get rid of once you have it.

lesser celandine...possibly 'Brazen Hussey' with the dark leaves

Thanks folks!

Once the flowers are past it disappears until next Spring. If you dig up a plant it has roots which run around underground (rhizomes). They look like fat little sausages. I don't worry about them but if I dig up any roots I tend to put them in my landfill bin.

Thanks Scotsgran - I'll pass this info on to my neighbour and leave it up to him whether he wants to nuture it or bin it!!