Just discovered that one of several claimants to be England's largest second-hand bookshop is in Rochester - Baggins' Book Bazaar. That could easily be got to and back from by train in a day, leaving plenty of time to explore the shop. I'l do that in the new year. Wigtown is a minimum of nine hours by train and bus, and probably costs the earth. Might be good for a week's holiday, but obviously not for a day trip. Hay-on-Wye, with the two largest shops in Wales, is similarly impractical to visit for a day. So - Rochester it is, in the new year.