Thanks all for the advice.
Had another go at starting this morning. Filled filter, primed and easy start but still no luck. Ran down 2 batteries with car just cranking. Decided to get the crud out of the fuel system so I removed the fuel tank, emptied and cleaned - quite a lot of dirty black bits in there!, and cleaned fuel pipes as best as I could. Reassembled system and tried again. No change. Then I got a fuel can with fresh diesel and a length of fuel pipe inside and connected it to the inlet on the filter housing. Raising the can higher than the filter to gravity feed I had another go. Five seconds of cranking and car started fine. Ran it for about 10 minutes with smooth revving and nice even tick over. Changed housing inlet back to proper fuel line and it started straight away but only ran for a short time before fuel starvation. Probably used what was in the filter. Decided then to bypass the Ravs filter system so I joined the housing inlet fuel pipe to the high pressure fuel pump pipe with an inline filter between them. Car started first turn of the key, I added fuel system cleaner to the diesel and took it for a 10 mile drive. It ran fine with good power and started immediately after stopping. Probably done another 20 miles since with no issues.
Can only assume there is a problem in the the filter housing. Diaphragm as suggested? Is it possible the the filter was so dirty that it backed up to the top of the housing causing a blockage? I don't know.
The fuel system I am running now is not ideal and as soon as I can get another filter housing I will put it back to the way it is supposed to be. At least I can keep an eye on dirt in the system as the inline filter is clear.
Again, help is much appreciated