For those that have got tickets the train is good value down.
I have just booked the 8.25 train from Moor Street to Marylebone 10.20am arrival for £26.75 including booking fee for an adult and 2 under 16. It is on the family travelcard (no special card needed) that also includes all tube's,buses, DLR etc for the day. It's an open return so you can come back as and when you want on the day.