Seen on our walk, I think it is Wild Plum blossom. 
I went to the Garden Centre this afternoon and bought some flower seeds, a pot of white Muscari and a reduced pot of Hellebore.
Continuing the saga of the magnet fishing, all the stuff that wasn't of interest to them has been dumped on the river bank and path for the rest of us to manoeuvre past, how selfish is that.