Hello @AndyE , and welcome to the forums I reckon May 1st is a really good day to join!
I reckon that you have something there that`s been grafted onto Willow stock - not sure what it is, but if it flowers we might have a better guess. The other leaves that look yellow/variegated look a bit like one of the Euonymus family....
I think that if you don`t cut off all the Willow growth (straight away) , there is a good chance you will lose whatever it is that`s on the graft.
This may have happened because the variegated shrub was pruned back too hard.
Have you seen any flowers on the top growth yet?
What a curious grafted willow.
The scion seems to be an unusual variegated form of Grey Willow (Salix cinerea 'Variegata').
I am not at all sure what the rootstock willow is, but it doesn't seem to be either Salix cinerea or Salix caprea.
Other likely candidates such as Salix purpurea and Salix viminalis don't seem quite right either, but I won't rule them out.