The sun came out today so I had a go at the problem and I have found the probable cause without even going underneath. I thought I would start under the bonnet (hood).
I removed the return pipe from the regulator and checked the flow, which was good. When the pump stopped after a few seconds the flow continued for a second or so and the pressure disappeared.
Unfortunately I broke the quick disconnect fitting which did not quick disconnect!! I have chopped it off and replaced it with a pipe.
Having removed the regulator I found no leakage through it , checked it with a low pressure air line.
I noticed there were two O rings on the reg. and I suspected the small one in the centre might be leaking so that pressure was bypassing the reg.
I did not have the correct one to replace it with so I fitted another one on top of what was there and put it back and started the engine again.
When I switched off, lo and behold the pressure did not drop. I left it for an hour and the drop was only about 2psi. So I think I have solved to problem, time will tell.
I did not expect that to be the cause of all that hassle. A tiny O ring costing no more than a couple of cents.