Sod's law at work.
First person to come for a viewing loved the car. I got in to reverse it out of the garage and ABS light and ESP light were both on. Got up to about 10 mph and all sorts of big flashing brake symbols and beeps.
Did a quick brake test and all was fine (well except for the abs not cutting in).
Plugged in my generic OBD2 reader and nothing found. Borrowed a VAG specific one and just got a generic looking code and a lack of contact with ABS control module. Having hoped for a wheel sensor this was looking serious.
Local independent garage specialising in Audi and Saab plugged into some grown up diagnostics - same message. Ahh, says mechanic, I've seen this before, it's either a lost 12v to the controller or a poor earth. Leave it with me.
two hours later he rings up and says sorted - the 12v line runs under the battery tray and the battery overflow dumps acid onto the loom causing corrosion, a voltage drop and eventually not enough voltage to switch the abs control module on so it won't talk to ECU or diagnostic kit. New wire spliced into loom, bill for £140 and we're all fixed. I was thinking of a new abs control module and about 5 hours labour to fit - so very happy.
bl**** modern cars with no access.