In reply to Joe Barlow:
don't know much about the train although I'm sure there is a bus, I've never taken it but looked at it a few years ago. think it used to leave from London Victoria. eurolines may have been the company.
personally I think Flying is the best option. cheap enough too if you book it far enough in advance.
You can also drive and avoid the tolls. Can't remember the route exactly but sure this involves going through Belgium initially.