On your diagnostics can you access live data from ECU?
To test the Fuel rail pressure, disconnect all the injector multi plugs and the inlet metering valve (brown plug on the 'pump') then crank for 5 secs. Fuel pressure should be 1600bar. It'll need 250 bar to start/idle with everything plugged in.
(with the IMV disconnected the pump should deliver it's maximum pressure)
Obviously you need to make sure the low pressure side is primed before doing this!
As others have said, hand primer on the filter normally works, I've never cracked off the injectors on a TDCI when trying to start it, never needed to. Surely that will draw air in..
If you have P0251 it could be a pump problem, but it could also be fault with injectors/air leaks/filter blocked/fuel pipes crimped.
With fuel low pressure side primed and fuel pressure tested, if the pressure is low you need to do a leak off test on the injectors.
(random google pic)
If you haven't got the fancy kit to test leak off, just get 4 EQUAL lengths of washer pipe that'll fit on the injector leak offs and 4 tubs/bottles/whatever that are transparent.
Again disconnect the injectors and inlet metering valve, and 5 sec crank and check if fuel is present in any of the hoses, if one has a load of fuel in, there's your faulty injector that is losing your fuel pressure!
If you can get it started, you can run it with the leak offs into the containers and run it for a minute to see how your leak offs compare.
When Transits and Mondeos had all the injector problems we had to ring Ford prior approval to get them to authorise the replacement under warranty, proper ball ache having to go through the results of all of the above, plus compression test results...
Ford prior approval : "whats your 1 min run test leak off quantity"
GVK - "Haven't done one"
Ford prior approval "Why not?"
GVK - "It fills the bottle in 30 secs, If I run it for a minute it'll piss diesel on the floor!
Ford prior approval "Oh, errr, I see - fit an injector in that cylinder then.."