Steve and barflux:
Only a few more observations. I was thinking along the same lines as Steve on using misfire detection, my only other thought Steve is that Barflux is in Great Britain, and I am unaware of what the engine management setup is there, it may be different in the diagnostic regime than a US (OBDII) vehicle. But hopefully they can go into an actual value screen and check out misfire recognition, even though the threshold for a warning lamp may not be there.
But another thought I have is about the "damaged condensor" problem that was repaired. Just how badly was it damaged, just wondering if it were in a major front end collision, maybe there are some chassis issue (misalignment of engine/driveline/exhaust, motor mounts) that may be causing a vibration or resonance causing the complaint.
And also on the noise from the AC and the fact it is now "bloody cold" I wonder if an overcharged system could cause the excess low temps and a hissing noise? Just a thought, trying to put 2 and 2 together here possibly, especially if both of these concerns happened after the "condensor damage" was repaired.
ps or another thought is maybe an overcharged AC system making the compressor turn harder, but not enough to make the belt slip, but laboring the engine and causing a driveability concern such as this? Maybe run with the climate control off for a while as a test to see if the driveablity concern disappears.