Well this is embarrassing. But good news for me. Thanks Moarpower, you were right, just a stuffed battery. The odd cranking behaviour made me assume something more complicated, but checking while cold the battery was 10.5 volts. Amazing it started at all really, though the battery was huge, over spec-d for the car, but nevertheless stuffed from sitting unused with a crude trickle-charger cooking it for months (car was in storage for ages before I bought it) Thanks for the reply!