On getting out of the car last night, I noticed a water leak, and was a bit concerned, as it clearly wasn't coming from any normal overflow pipe.
I had a quick look, but couldn't see exactly where it was coming from. It's not the water neck (prime suspect due to recent change), but was dripping (quite rapidly) from the crank area at the front. I released the pressure and the leak stopped.
I tried to get it going again (topped up water and left her running), but temp wouldn't go high enough on idle for the pressure to build up enough, so I will have another look tonight after a drive to get the temps/pressure up.
My initial thoughts are the water pump. Is it possible that these leak, or is it more likely to be the seal to the block gone?
Whatever it is, it's more than likely been due to my changing the rad cap/water neck and thermostat and the pressure returning to what it should be has busted something.