Ok, so the AA guy plugged in his diagnostic computer, which reported "P0100 Mass or airflow "A" circuit", and "P0110 Intake air temp sensor 1 circuit". Not surprising as the MAF was disconnected. No other faults.
After consulting the AA mothership he was able to figure out the wiring to connect the MAF back up, cleared the fault codes, and everything seemed fine.
He did notice that when I was revving it hard, the intake pipe was actually imploding due to vacuum - thought it might point to a clogged air filter, and that might have caused the engine warning in the first place if the ECU thought it wasn't getting any air.
Anyway, back on the road I realised it was still down on power, not pulling at all in 6th or 5th. At one point also I glanced in the mirror and saw a huge cloud of smoke dissipating behind me - think it was as I downshifted after having my foot flat in 6th and making no progress, so I guess coughing out unburnt fuel?
Power seemed to get worse and eventually the engine light came back on, and we just limped the last couple of miles to the in-laws. Hoping it'll get back to London in limp-home mode tomorrow, and then it'll be up to Autolusso as soon as I can sort that out.
I'm supposed to be collecting my teledials Monday evening too. Maybe should have saved that money, might have gone toward sorting this out ;-)