Well I finally graduated to become a fully fledged P38 owner today. I have undergone the right of passage and got a lift home on the back of a flat-bed!
The cause of this was a failed accessories drive belt. The very belt that I replaced as a preventative measure when I first got the vehicle whilst I had the radiator out barely two months ago!
All effected pulleys and accessories have been checked and are free to rotate. Alignment looks to be correct too.
Joy!