This has been an ongoing thing for several years and was formerly know as "The Amsterdam Express". Travelling from any "ONE" station in East Anglia, including from Liverpool Street, London, taking you to any Dutch station, Amsterdam or further afield, AND including the ferry crossing from Harwich, the 25 quid deal is definitely worth a thought - OK it takes a bit longer, but without the hassle of airline check-ins, baggage restrictions and etc. - this is a nice way to travel if time is not a commodity.
There seems to be no restriction on dates, but you must book online at least 7 days in advance.
