Just a quick thought, my 2p.
I had a cracked windscreen in George a couple of months ago. When it was being replaced the guy insisted on disconnecting the battery - "health and safety" - no idea why.
Afterwards George ran rough for a little while. I checked out the handy Haynes book and it suggested that removing power resets the ECU, and that it has to "relearn" its settings - fuel quality, timing etc. Mr Haynes recommended a quick blast at different speeds for about 20 minutes and its been fine since.
Just wondered if a long period of inactivity - even if the battery was still in the car - if the same thing happened ?
The cat on George is the next bit of pipe from the manifold which is lucky because the rear silencers have a reputation for rusting very quickly
