Update.
Sorry for the late reply, got it all sorted now. I took the car to the garage who changed the pump. Naturally they rejected that it was the pump before they conduct tests of their own. I supervised whilst they were conducting the tests to see where the leak was coming from. It was infact coming from the thermostat and not the pump.
So they replaced the thermostat. And retested for leaks, and none were apparant. I asked if leaking fluid would damage the belts. The mechanic responded with what everyone else said, that it does. However, he was confident that due to the position of the thermostat, and that the belts were half obscured by another componant, no fluid actually leaked onto the belts. Which meant not needing to replace them.
Naturally I'm sceptical about this. But, since the fault of the leak wasn't the water pump but rather the thermostat, it means for them to replace the belts, it would cost me rather than them. So, I've decided to play the risk game and see what happens.
I've taken the car for a blast on A roads, and in traffic stop-starts, and the temps seem very stable - i.e. don't move at all from the middle.