Thanks Maggie but I think that Jacek has it.
One of the weedy Solidago? Leaves seem to be exactly like Solidago canadensis, but there are too many branches on the top. This is a weed, but one of those nice ones.
Obvious really - thanks!
The reason that it is obvious is that there was loads of Solidago over the other side of the garden when I moved in. They also grew teasels, Forget me Nots and all manner of other weeds. Worse still, they put everything in the compost so even after I dug everything out when I innocently used their compost I was spreading it all around again!
This will be in the bin later!