The quantity of fuel required is very small - each HP fuel rail holds 36cc, an injector passing excessive amounts back to tank would certainly cause it to cut out whilst running - unfortunately there were too many goes on the parts dartboard to diagnose what the root cause was at this stage.
The non-return valve being open would not give you fuel 'backing up' in the return line, the valve being closed, or blocked would cause excessive pressure in the return line, in turn preventing flow of fuel if it were greater or equal to the LP input pressure.

TBH, this is all basic stuff, applicable to fuel systems of earlier designs and implementations.
Having worked for LR dealers, they wouldn't / shouldn't say they don't want to look at it.