Hi James, I had exactly this type of fault on a Mark 4 just before Christmas, could you please post exact fault codes as this will give a clearer picture.
Golfs are prone to getting water in the ECU so will be worth checking this as well, but mine turned out to be a faulty wire from system relay (109) ? it had continuity but would not carry current, by grounding the relevent pin (49 in your case I think) it would start, or alternatively by powering pin 87 at relay.
With both ends of the wire disconnected and 12v probe applied I was only getting 7.5v out, running a new wire cured the problem, the old one must have been hanging on by a few strands although it was well protected in a large loom.
Hope this helps but sods law says you will probably have a different fault!!
I`m sure someone else on here will be able to give some help, good luck Roger.