"Say Hello and step in" ..OK, long time not here, but was asked to look over.
What for me is curious is that there is a complete bank of injectors given as inactive.
So, as Mike has already mentioned, the Engine Injector system (engine harness) is feed ground on the bellhousing, and there oxids of steady salty UK weather driving are a bad thing.
My additional tought on this would be:
the engine block itself needs a solid ground as well ..this goes to chassis and should also be free from oxids on the contact surface, of course
to proove the integrity of all small wires of the engine system (ECM) undo the multi-contact ECM connector, grab a small needle or paper clip .. and use the workshopbook for harness details ..this way you can first check that all wiring on its own is intact and usable
for test of "injector feed" from ECM use simple test lights, who mimic an real injector, those are available in garages and should be there to find for right type of injector (different voltage / connector types are in use in automotive world)
for verification of fuel pressure a simple hydraulic pressure tester is usable, feed via the Shraeder valve connector on fuel rail (RH side)
if one of the mentioned fuelpumps is old (normally after around 20 years should be the main pump, the one that does have to run steady) - just bridge the relay that is activated on overboost and on "start up" .. as only ECM activates this second fuel-pump normally. If you activate this pump manually by manipulating the contact the second fuelpump will run even on idle and medium cruise, and hold the fuel delivery on top level
if you do this manipulation simply by unplugging of relay, the ECM of course will detect an additional fault and set a code most times
it is therefore worth to just swap the pumps within wiring iself - or swapp them over physically in the fuel tank (two pumps sit next to each other in a bracket (holder) in the tank on RH side, both pumps are technically the same)
..this for now - have to sleep some night on it
SOLERA Holding is my new employer in Germany, US guys may know what this means mentally ..