How can we get a refund for a mouldy rental property?

Q I have spent three weeks trying to resolve a case of an extremely substandard rental property, with mould and insects in the kitchen (and the wifi didn’t work).

We left the property within 15 minutes and found somewhere else to stay, but the host is refusing to give me a refund. They claim they cleaned the property before we arrived. You’d expect the booking agent to help out, right? But Booking.​com say it’s nothing to do with them. They have given me a £37 credit on my account. But is there anything I can do to get a full refund?

Mark P