200tdi fuel issue

Hi, been having a few problems with my defender. It has a 200tdi disco engine in it. It has been really underpowered which is especially noticeable when pulling away as the engine really struggles. When starting it cold it takes a while to fire. It cranks over a few times, fires but doesnt catch, cranks over several more times fire again but doesnt run until eventually it finally runs. After speaking to a mechanic he said it might be the lift pump, so earlier today i replaced that. After doing this however the engine is worse than ever. It still has the issue starting and now it struggles on tickover and then stalls. if you rev the engine it takes a while to respond, revs and then stalls. obviously the vehicle is now undriveable. also noticed that there appeared to be gas in the fuel tank that looked like exhaust that could be seen down the filler pipe. Any suggestions? Cheers in advance.

has it allways been like this since engine fitted or just come on,sounds like injection pump is not getting enough fuel and too much air ,as these are self bleeding they will run with air in pump but not properly,undo front pipe on injection pump and using lift pump see if ample fuel with no bubbles is present,,tank ,pipes lift pump okay ,,remove solenoid and remove plunger refit and try running you may need to gring down 15/16 open ended spanner to remove ,it may be poorly injection pump

retro fitted the engine a year ago and it was fine. the engine then sat around for a few months while i rebuilt it and eva since then t hasnt run right, so the last months or so.