Sissington Castle / Leeds Castle

Best travel options (from London)?
I think we're (mom & dad) thinking of day trips... so just want good transport links, preferably without too many stairs or too much walking to get to/from.

Suggestions? 

I've been to Leeds but by driving (no stairs and very little walking!) but the website, http://www.leedscastle.org.uk/, has National Express coach and Southeastern trains/bus options.  I was meeting friends from London there and I think they did the National Express which leaves at 9:00 from Victoria and arrives at the castle at 10:25, they said it was pretty good.

I've not been to Sissington, don't know where it is?

Leeds Castle is a lovely day trip from London but we did it by car.  It might be worth hiring a car for the day to drive over there.  There aren't a lot of stairs that I recall, and the actual castle is a bit disappointing inside.  But the gardens are lovely and we got seriously lost in the maze.  My ex enjoyed the bird show but considering my bird phobia, I didn't want to get too close.  It was a worthwhile day out.  I'd go again.