] The book tells us that The Abbey is located about thirty miles from Bath, and that Petty France is about halfway.
] Does anyone know of an ex-convent of nuns in that area?

Not an abbey, but what about Berkeley castle? Still inhabited and formerly a royal castle, there was also the famous Dr Jenner living in the village...

There is always the famous Malmesbury Abbey, but according to the website by the 18th century it had decayed to the extent that it was being used to store hay and pigs.
http://www.malmesburyabbey.com/