S80 D5 2003 Car will not start

[Bionic-Bulldog]

i am having strange problem that keeps coming and going.

the other day car would not start for the first time ever, just turned over like it was getting no diesel. called out the RAC who ran diagnostic checks with a laptop - no faults were found. he checked fuel pump- all ok, diesel was getting through. he checked various other things all which he said were ok. he then came to the fuse box and checked the ECU and Glow plug relays. when he removed them and put them back in. all of a sudden the car started. Ahhh dicky relays we thought, problem solved.

however, now i am a few days from that visit, the problem is back, i checked the same relays only to find them appearing to operate perfectly. seems it was pure luck/coincidence that the car started after Mr RAC man fiddled with them.

what has me puzzled, is that at the same time the fault appears, my automatic gears shift lights also do not light up,(ie. park light no longer comes on when ignition is turned on and the gear in use indication on the dashboard display just shows - instead of the usual P etc. ). also whatever is causing the issue over-ides the "push brake to move automatic gear shift", and i can move the lever into drive etc without pushing the brake.

These two relays that we first thought were the problem, appear to have no connection with the gears shift at all. When the car was working, i put the ignition into i position so the gear lights were on, then went and pulled out the two relays from the engine fuse box. Gear light remained on.

whatever is causing the issue is obviously not these relays, but i can find no schematic that will enable me to find a common denominator between the gear shift wiring and the car not starting.

any ideas anyone? sorry for the long post, but trying to put in everything that has been tried and found so far.