Before I drove the Golf to the garage I topped the coolant bottle up with water and I didn’t get any warnings all the way there.
I’ve just had a call from the garage who tell me they have had it running for the best part of 2 hours (I’ll be needing diesel then) and they can’t see it leaking anywhere. I’ve asked them to drain the coolant, refill it and put it on the rolling road. Turns out they’ve only got a break tester but I’m waiting for a further call to see what they can do.