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?