PS it isn't as big as the photo makes it appear! Could it be a Mimulus? I hope so a the ones round the edge of the pond got mown down last year. Well he was doing his best...

before I saw your comment I thought mimulus.
so I think you have it cracked so to speak. I'd certainly say it was mimulus.

Yes, I would say mimulus. I have some self seeded ones looking very similar.

Mimulus x hybridus, Stera. Some species of Mimulus are perennials, but this one is bred from high altitude tropical species, so probably better as an annual in the UK.

Thanks everyone! Tug they were doing fine as a perennial until my dear OH mowed right up to the edge of the pond and killed them all...So this self sown one a long way from the pond is all I have left...Must try to save seed and start again!

Oh...maybe Mimulus guttatus, then. I thought that the leaves looked more like hybridus, but I really wasn't seeing them as well as I thought. The flattened flowers should have given me a clue.

Well! Thank you all. Armed with your remarks I've just been out again (in the lovely welcome rain"!)and had another look. After looking up these options in three different books and measuring the height etc I think it must indeed be guttatus - apparently this was first naturalised in Wales although come to think of it now I remember seeing it growing in profusion on the edge of a lake in Staffordshire and thinking it looked like Mimulus but had no idea it grew wild so thought it must be something else

I just hope I can manage to dig it up later from where it is wedged in the crevice under the the step and transfer it to where it was in the first place! (With a KEEP OFF DO NOT MOW notice for OH!)