Fuel filter issues

[Qldmts]

I have a 2006 transit . It’s the first 2.4 TDci mk6 common rail model with Delphi system.
I replaced the fuel pump and injectors 9000 Kim’s ago and cleaned or replaced the rest of the system so everything was spotless so all would go well. Over the last weeks I’ve been getting the same old alarms that I got that made me change out the units mentioned. The difference this time is the pump pressure is good. I did a leak off test and found #3 injector was bad so I replaced it. They said after inspection that it showed wear. #2 & 4 were the next ones that seemed to be leaking more than they should. Today after a trip it won’t start and when I do get it to run I get excessive leak back from #2 & 4.
Before I go changing these I am concerned that the filter is not doing it’s job and has let contamination in. I think about a month ago I got some bad fuel as that is when I noticed the idle getting rough.

I am told to fit a 2 micron filter between the existing filter and injector pump as the return line feeds fuel back into the filter housing but not on the pre filter side so if there is contamination it just keeps going round and round.
A filter in that position ensures all fuel is filtered.
Anyone got any suggestions or had similar issues?