Sorry for not updating this (been offline for a while).

After checking everything else we decided to pull the injectors(came out very easily), and got sent to be checked.
Results were all 4 were blocked with contaminated fuel.
So following a talk with customer and advice from company that tested injectors the following was done.

Reconned injectors.

New rail, pipes and filter.

Tank plus pipes removed and flushed out.

High pressure pump removed and tested (OK).

After all that the vehicle now been running around for nearly a year with no issues.