As I suspected, Kwik-Fit were speaking out their !Removed!.
Popped into a local garage after seeing that they did AC Work on the off chance they could squeeze me in.
All absolutely fine, held a vacuum and no evidence of the dye anywhere. Filled 'er up and it's taken - been asked to check it works everyday for a week, and to pop back in just to make sure. They're also going to see if they can find the coolant leak I didn't realise I had :-(
(Only small, but the mechanic noticed the whiff of coolant when he popped the bonnet and the level has definitely dropped)
Anywhoo... £54 later and I need a coat on in the car