Thanks for the reply - I really appreciate the interest. I absolutely agree that the coolant pressurising and the metal flakes are completely unrelated. If it weren't for seeing them in the filter, I would be quite happy thinking new head gaskets and away we go. Regarding the coolant system, when I first got the car as an overheating runner (about 2 years ago) I checked everything else hoping it was an air lock/bleeding issue but it soon became obvious that it wasn't - probably the main reason its just sat there for so long as other things in life have taken priority but recently I decided it was time to do something about it. When removing the exhaust system, there was actually coolant drips coming out of the manifold so there has definitely been coolant getting in there (presumably when the engine gets switched off and there are no longer combustion gases forcing into the coolant system).
The bits in the filter are definitely metal and definitely non magnetic. They are very fine but there is a LOT of them - I will try and get a picture. I would say they look to be coppery but of course they could just be oil stained.
Hopefully I can do a bit more investigating this weekend and get a bit closer to finding out. On a side note, whilst I know the gauges arent exactly reliable, the oil pressure was showing absolutely fine on the dash prior to this strip down.