Is National Trust membership worth having?

Every week a Guardian Money reader submits a question, and it's up to you to help him or her out – a selection of the best answers will appear in next Saturday's paper. This week's question: Last week we bought joint National Trust membership. I'm not sure if we'll really use it – we live and work in London, which doesn't have many NT homes. Do most people ditch membership after a year? What's the best way to get value out of it? What are your thoughts?