Hi Roy.
Try getting it on a T4 which could give you a good idea of what's going on, another thing to look at is doing a leak back test on the injectors, also the the HPFP regulator O-rings these can sometimes leak, cheap fix maybe about £5.25 and an hours work to fit them.
https://www.ebay.co.uk/itm/333828232...m=333828232470
Another thing to check is the wiring plug on the end of the fuel rail, remove it clean with contact cleaner, & push it on and off a few time, sometimes the wiring can have an hair line crack and may need replacing.
But T4 is the first port of call to save guessing, i think Big Russ is only an hour from you.
PS have the plenums been checked for water maybe the ECU as been for a swim?
https://www.the75andztclub.co.uk/for...d.php?t=125864