Thought I should post my progress.
I did a bit more research and found that some other Merc diesels had the same problem with the owners replacing or cleaning the MAF sensor. I decided I would give it a go.
Went to a local motor factors and got some Holts Electrical contact cleaner (£4.99). Went to halfords and got a security torx key set to unbolt the MAF sensor (£9.85). Unbolted it to find the sensor covered in a layer of black crap. All in the recesses and all over the contacts. obviously, 90k miles and filter not changed in atleast 35k (blame the missus
) hasn't done it any good.
Got a few cotton buds and with the contact cleaner in hand, set to work cleaning it up. Sprayed the cleaner over the lot, let it soak for about 20 seconds and began to clean up the contacts and bare metal areas, as well as all of the plastic parts. Also spayed the cleaner into all of the recesses and tried my best to clean out all of the black crap from inside them.
Got it looking good as new and popped it back in.
The car has been transformed! Pulls far better than it ever has and as of yet, has not lost power again. Revs clean upto the 4.5k limit and pulls like a train. Ive done about 20 miles in it since cleaning it and it hasnt lost power again so far where as yesterday, it would do it on every drive after a few miles.
The missus is off to manchester tomorrow so fingers crossed, the power loss wont come back!
So just to say really, anyone that has this problem, pop the MAF out, give it a good clean with contact cleaner and go from there! Seems silly to replace a fine MAF for a £160 new one when all it needs is a bit of a clean. Worth a go first anyway.
It also makes sense as to why the error light never comes on. Sensor is perfectly fine. Not faulty in anyway, its just that the ECU will be getting wrong readings and so shuts itself down into limp home mode but doesnt display the light as nothing is actually broken.