Good news - it was of all things just the negative battery clamp not attached to the post properly - i.e. literally just sitting on the last 1mm of the post! Classic case of always check the basics first... Looks like the terminal clamp slipped up as it was tightened up - and to add insult to injury, the battery clamp and bolt were just sitting in place too. Someone had a shocker!

Thanks again all, loving the car and VWWC!