It would be very simple to install an in-line filter in place of the existing fuel filter as a temporary measure. It certainly won't cost £300 :yuck:
It would eliminate the fuel filter as a symptom immediately. If you are prepared to take a (small) risk, bypass the fuel filter with a piece of fuel hose and run the vehicle for a short while.
If the vehicle runs all day (sometimes) then the fuel filter is most likely NOT the cause.
It could be the stop solenoid (quite likely - a simple loose connection).
It could be electrical (loose wiring)
It could be the fuel pump (unlikely)
Any garage that wants to do 'process of elimination' tests that cost £300 a shot should be avoided.
Try to get hold of someone familiar with diesels that can come out with you - you might get a chap to do it for a back-hander (£50 say) - who can trace the fault when it occurs. If you take the van to the garage and the van is running they can't be blamed for not finding a fault that isn't there!