Just to update on the outcome; I contacted the guest the morning after being on here, told them I had been on to AirBnB and that they could cancel and receive some refund. It was 2 days into their 'stay' at that stage, so they lost those 2 days, plus the following night. This was midway through the total stay, so I received roughly half my original total, which I was happy with.
The ABnB guy did say I could do nothing, in which case the guest would have got nothing as they didn't seem to know about the 'Cancel' policy, but it didn't sit right with me.
So, I have a lovely clean home, some family took the room and am happy with the outcome.
General point - I've read lots of negatives about the support from ABnB but have to say I find the customer service great. Between the community advice here (I read and search. Don't need to ask so much) and the 'formal' support, touch wood, I'm really happy with the whole ABB experience. My thanks!