This is a word of thanks for the replies, and a report. We had a great time. Stayed in Folkstone and liked it there - it's definitely more pleasant than Dover. We went to Dover by rail on both days, going to St Margaret's Bay by bus the first day, which is a wonderful place to walk along the base of the cliffs on the beach - also had delicious smoked mackeral there. Did the castle later that day, which is also worthwhile. The second day we went out onto the pier at Dover to see the cliffs from afar, and then spent the afternoon walking along the tops of the cliffs in the park to the east of the castle - gorgeous country and spectacular views!
RH