Rufford Abbey is a former Cistercian abbey near Ollerton in Nottinghamshire, founded in 1147. English Heritage looks after the remains, which include the best-preserved west cloister range of a Cistercian abbey in England, dating mainly from around 1170. After the dissolution it was converted into a country house, and the abbey remains are built into part of a 17th century and later mansion.
Tip: Check the English Heritage and Rufford Abbey Country Park websites before you go, as access to the inside of the ruins can change and the car park is run separately from English Heritage. Dogs on leads are welcome.