The azalea has also turned up but a bit late. No sign of hostas.
I was trying to find out a bit more detail about why trees send out side shoots - whether there was a particular reason or stress in general. One of the reasons the RHS cited was high water table, and I think that is a problem here. It fits with the more successful trees being willows and the like.
That's also a bit of a clue for me, because when planting new things I can add soak-aways to divert water away, add gravel etc to the soil immediately around and, of course, choose the ones with suitable roots. I'd been thinking the problem was above ground and hadn't been thinking enough about below.