CR170 Hard to start after replaced fuel tank.

Feeling frustrated.

Following on from previous post.

2014 Superb Elegance TDi CR170

Back story.

Ran over something in the road, punched a hole in the fuel tank. Mechanic replaced the tank with one I sourced from a breaker that included pump, gauge sender etc.

Car was then hard to start. Mech thought maybe low fuel, so put half a tank in. Following day, it started but cranked for ages before it started. Drove fine for a while then gave me limp mode, flashing glow plug light etc. Code comes back as P008700 - fuel rail/system pressure too low. Checked getting fuel out of tank pump, and at filter housing (and was full to top), HPFP and rail but I have no idea if any of those are ‘in spec’. Fuel filter had 15,000km (9000 miles) on it so well within factory intervals but changed it anyways just in case but noted this time the housing was nearly empty. No sign of HPFP metal pieces. Still very hard to start if sitting for more than a minute but runs ok til you give it some stick - brings on limp mode about 4.5k rpm. Im usually pretty sedate driver, but tow a trailer fairly regularly so gave it some death to make sure it’s ok which its clearly not.

Question is - could the tank pump be causing my issues? I have the old tank with pump, so can change it but I don’t know if it’s my problem. Otherwise it goes back to the mechanic on Monday.

TIA