Breathing/fueling/rad fan problem??? 2009 JCW

Hi guys,

Can anybody shed any light on this problem??

I've got a late 2009 factory JCW and this problem started intermittently about 6 months ago but now seems to be permanent!

Symptoms are:

Jerky acceleration (hold throttle in steady position, yet feels as though you're opening and closing very quickly)
Induction noise has changed for the worse. Sounds louder than usual and just generally like it's poorly, doesn't seem to be the exhaust though.
The "pops" you get on the overrun have changed from the usual single pop to several pops
Power is down quite dramatically - feels strangled
Radiator fan often comes on, even after doing a very short journey, and stays on after turning the engine off. Impossible that it's had the chance to get warm, let alone too hot.

I actually had it serviced straight after the problem started and the garage said it was the plugs - which I had sorted.

Just had a look at the air filter to see if that was okay, and it seems fine. No sign of blockages or anything simple.

Any help would of course be appreciated, as I want to put the car up for sale this month (sadly :-( company car has now arrived and the mini is in the way :-( :-( :-( )

Cheers
James