An update on the air filter problem...
I cleaned the air filter (it turned out that I had a BMC Carbon Dynamic Airbox filter) using recommended oil. However, there was very little performance improvement, if any at all. Accelerating to 3,000rpm and above had always been a struggle, and still was.
The air intake pipe coming form the bottom of the filter box had snapped! Leaving it hanging down, rather than be facing the front of the car to get the cool air. So, a friend of mine fited a new air intake pipe to the bottom of the filter box and secured it to the front grill.
No immediate improvement. BUT, and a this is a big BUT... I was driving down the motorway at roughly 80 - 90 mph for an hour or so before I left the motorway to hit an A road. I tried to overtake a caravan in 3rd gear, and all of a sudden the clutch started slipping, before regaining its grip and accelerating. When ever I put my foot down now in third or fourth gear and the revs hit 2,000 rpm (i.e. when the turbo should kick in) the clutch slips, revs go to about 3,000 - 3,5000 rpm before finally regaining grip.
On the plus side though, my car is now rapid!! It's as if my turbo is back (maybe a blockage has been cleared??) The car flies in 1st and second gears, with the turbo better than ever before. But when I stick the car into third gear, it slips again when the turbo should be kicking in (i.e. 2,000 rpm). The clutch does not slip in 1st or second gears....
Forgive my ignorance here, but does this sound like a clutch problem or something else? I am scared to take it to get the clutch repaired only to find the car is a slug again