As a caveat i'm not sure whether what i'm about to say has any legal standing, but here goes...
Before they took over they had the books and should be aware of all revenue and expenses vis a vis the pub, including your £48 bill every 2 months. That is after all £288, no small beer. If they took over in September last year and you have cleaned it since then (say in November and February, and have been paid by them then I would have thought that any court might agree that there was a tacit continuation of service - they could have cancelled you after all. A contract would be implied.
I would personally write a final letter outlining the above, saying that you accept that they no longer want your services and you have cancelled them, but that their bill is still outstanding; give them 30 days to pay; if they don't pay I'd get Thos Higgins involved - a Letter Before Action costs £2.40. If they still don't pay, forget it.
Best of luck.