I took the water pump off and had a good look at it.
After a bit of a fiasco (access, nuts, metric and imperial issues between the pumps) I then noticed that the pulley was worn....
a closer look still and the pulley rubs on the nuts of the pump (the nuts are worn). The new water pump is about 3mm thicker which would create a few problems.
Here is the old and new pump and the pulley.
Still on the dining room table rather than in situ working away. Tomorrow the grinder will get wielded at the pulley and possibly on the pump (not the important parts like the mating surfaces!). Hopefully all will go to plan and I will have everything back together in no time (no laughing at the back please).
When I picked up the new pump I was told it was quite rare. With this is mind, once this debacle is over I will be looking to refurbish the old one.... and all help is welcome!
I am now in the pub drinking gin. All spelling and grammar mistakes are my very own.