mystery non starter

I have eventually got my range rover 2.5 diesel into a position to sell.
Various things completed including alternator, brakes, air springs M.O.T ect
it started perfectly yesterday and all throughout the day.
Today, however with all the 'for sale' signs in the windows a person came to look at it and embarrassingly it didn't start. The battery appeared to be flat.
I tried to start with jump leads and for some reason, lots of cranking but no starting. i can hear the internal fuel pump working so presumably the fuel is going to the engine.
What could it possibly be? since coming back from the garage it has worked fine until now.
Very frustrating but to be expected from these vehicles.

Flat battery is usually the RF receiver waking up the BECM. After a flat battery, it's possible it has suffered loss of sync between the BECM and the engine ECU. That will need a syncmate or diagnostics. They don't like jump starts or flat batteries.

it has been jump started plenty of times throughout the 'renovation period' with no problems. it only required the security code recently. i will see if i can get a diagnostics test done. Would the engine still crank if it there is a loss of sync?

Yes but won't let the fip
Work