Want to go away for an evening, somewhere out of London, preferably in the country but could do a city break, has to be via public transport though.

I'm thinking catching a train out of town on a Saturday morning, spending the afternoon there, meal out, evening in a nice hotel or B&B, bit of pottering Sunday, train back. Must be relatively cheap.

I thought about Brighton, but even off season the hotels are pricy, and I'm not sure about the seaside anyway. Maybe Bristol or Bath? I guess there'd be stuff to do there. Any other ideas?