Thank you everyone for all the fab advice! We've now booked Eurostar return (with the Disney Express service) and it only came in at around £200. We've got a Premier Inn booked for an overnight in London, and so far, the 12 week advance train tickets from Manchester to London Euston work out at about £40 return! I'm planning ahead to minimise luggage! (why take 3 forms of shower gel/shampoo when everyone can use baby top-to-toe wash....And I KNOW they supply shower gel/shampoo, but, like any normal fan, they go straight into my bag to be added to my Disney collection at home...

).
You've all been a great help, and I'm sure I'll hear from you on other message threads before we go!