I travelled to Battle by train, my preferred mode of transport. There is a good service from London, with trains departing generally from Charing Cross, although they also stop at London Bridge. Travel time is between one hour twenty and one hour thirty from Charing Cross, slightly less from London Bridge. Trains leave London at half hourly intervals.
As always, use the website shown to plan your journey.
Battle is the perfect destination for a daytrip from Hastings, especially if you are into all the 1066 stuff. Battle is located on the Hastings-London trainline and during daytime, there's a train at least once an hour. It takes only 15 minutes from Hastings to Battle (William needed some more time back in 1066...) and a so-called "cheap return ticket" costs 2,80 pounds (July 2006).
There are frequent trains to London (Charing Cross) from Battle and in the opposite direction to Hastings.
It's a nice little station - you can even get tea and coffee there sometimes. It's old and quaint - like Battle itself. Don't expect to be able to get a taxi late at night though - we found a poor lost student doctor waiting there at around 11pm one night - He thought a taxi would just turn up - they don't at that time of night - you have to book one! So we gave him a lift anyway.