I think the issue was the relief valve. It was tight in the bore. Not rock solid, but it didn't slide out on its own.
It's a nice sliding fit now.
So I have just fired up the motor and it's running, without any horrible knocking or rattles, but it needs proper ignition timing setting.
Riddle me this: I thought the oil pressure should limit at 28 or 30 PSI and at idle I should see 10 or maybe a bit less.
When I first got the car, I am sure this is what I was seeing.
Now I am getting 34 PSI at about 1500 RPM and even at idle it's 25 PSI. (EDIT: Maybe this is just because it hasn't reached normal operating temp yet...)
Looking at my oil pump there is a warning pressure switch and a gauge BUT they come off different places. The warning light goes nowhere as there isn't a light on the dash that I can see.
Is it possible that the gauge is reading pressure before the relief valve?
My pump is an uprated one I believe. Not with the add on sandwich block, but I think its a later pump as the gears are quite deep.
Maybe this is how this sort of pump should run and before the relief valve was dropping the pressure all the time? It just got more stuck in the wrong place and completely dumped all the pressure...
I'm running some Morris Golden Film SAE 30 mineral oil through it at the moment to flush the old stuff out and I'll replace that with 10W40.
(Edit: Will set timing and run longer so oil gets warm and monitor pressure tomorrow!)
There's where the oil pump gauge and warning switch are plumbed in: