So today... I disconnected each injector in turn (again) to see if I could hear a change, which I couldn't. I then went down the route of insufficient fuel and tried what Jeremy suggested supplying the injector pump directly this made no difference.
I then removed one of the injectors tested it on a injector pressure tester (from ebay, not the best! but confirmed it was still firing at the right pressure) and connected it to an output of the injection pump. My logic was that if the pump output was running at the correct pressure I should see atomised fuel. I didn't, I didn't see any fuel at all. Maybe my logic is not so good or something wrong?
I then googled the lucas CAV dpa injector pump because I remembered reading about a 'last chance filter', from this I removed and cleaned the filter in the intake of the fuel pump which was surprisingly clean for an old machine! However when I reconnected everything, bled and tried to start the machine, it really struggled to start.
I cant see that anything that I did would have had an effect but it has now gone from starting instantly to really struggling.
The background- turns out if you keep topping up with water thinking "I must put some antifreeze coolant in" and never doing it. The engine freezes. It cracked the block along the side and top surface. So I got a recon block that had been crack and pressure tested and Then rebuilt, replacing all gaskets, front and rear oil seals, linners, pistons, rings, all bearings, valves, valve guides.
Valve clearances were set on rebuild and I have checked them in the last few days.
My thoughts now are pointing to the injection pump??
Thanks again guys for your time.
Dave