A blanking plate for the EGR valve won't do a thing until you sort out what's causing the limp mode, but will help after.

The usual cause is the intake manifold swirl flaps cankered up and/or the actuator diaphragm split. In your case I'd be inclined to say its the flaps in the manifold due to the carbonised oil you found in the EGR valve.

You will need to remove the inlet manifold which in itself is a right PITA, then either try clean off and free off the flaps, intake ports etc, or just replace the manifold if its too far gone. Personally, I'd just go for a straight replacement as its around a 3 hour job to swap out the manifold and you'll need to do it all over again if cleaning doesn't fix it.

Its a shame you don't want to pay for diagnosis time as the above advice is academic if the cause is something else entirely and and hour spent carrying out diagnosis and active tests may save you money and time rather than just throwing parts that may not be needed at it !

thanks to all who have replied.

Bit the bullet and took it to an auto electric specialist who saw the DPF was about 75% and recommended an active regen which I opted for.

after 30mins of running engine at over 3k revs and getting temp up to over 600degs to clear out the filter it seems to have worked.

Had 3 days of motorway driving with no reoccurance of the fault whereas before it was coming on every few miles.

Athough since yesterday the alternator light keeps coming on but goes out when engine switched off and on. Need to get checked as the alternator is less than a year old. I've got another battery that I'll swap over to rule that out... or not.

once again, thanks