[Dancingdad]

Why are you swapping sensors?
The sensor is telling the engine there is a fault.
That fault is that the ECU is not seeing the fuel pressure in the rail that it expects.
Which could be as simple as a clogged fuel filter or more complex such as leaky injectors, fuel pressure regulator or FUBAR HP fuel pump.

It is often worth checking live data with diagnostics...can save you playing parts darts with guesses or false assumptions.

 

[Dancingdad]

Ok, you seem to have covered most common issues.
But would still recommend plugging your diagnostics in when driving and if possible, get some live data.
My suspicion is that fuel flow is being compromised and that is leading to your issue.

Re the filter...... We have seen many cases where aftermarket filters have been fitted and this causes issues.
Often as the connection pipes are not quite right so the plug on connectors don't seal properly.
This can lead to air getting in...your little bubbles?????
Grease the connections to help the seals.

Injectors...leak off test will show if one at fault..... you can do this yourself.
Many different flavours of 1.5DCi and injector models can vary.... if replacing match the numbers, will also need coding in the ECU..... don't go there unless needed

 

[Dancingdad]

Can't really say except yes to the the above.
Mann is a good make as long as it is a genuine filter and not one simply called Man but nothing to do with the company.
If memory serves, seals are small O rings and these can get brittle and hard with age, even split....can be dug out and replaced.
And grease will help seal though whether more then a temporary fix depends on exact issue.
Should at least prove a point.