2a Diesel cuts out on cold start

Hi I have a 1964 series 2a diesel, it's been running well but today I jumped in it to go to work and it started fine then after about 5 secs cut out and wouldnt restart.
Later on in the day I tried it again and it did the same, it seems like fuel run back to me as I've undone an injector pipe and there isn't much diesel there, eventually I got it running and it restarts fine, but after it goes cold it plays back up.
I suspect the lift pump?? As it runs fine when it's running or a blockage? What do you think? Have changed the filter 2 months pervious to this which was very badly clogged up do they have a sediment catcher? Thanks in advance Jack

lift pump first then check pipe works not blocken in tank or sucking air in, check fuel stop is stopping in run position

Cheers I'll check it out tomorrow as I have 3 days off

after it stops try bleeding it at the sedimentor and injector pump if it starts straight away most likely air in system

Cheer Steve I think it is air in the system suspect it leaking through copper washers on injector pump going to replace them all today as the ones I have looked at so far are all in bad condition, the lift pump is ok as I have removed and checked that so I'll see how it starts in the morning cheers for advice