I booked a hotel package and eurostar separately for the Cheyenne in July and saved £400 on 2 adults and 1 child. I would definately recommend a separate booking - if you are wary of doing this your TA will do this for you (they usually ring both at the same time and confirm with you before booking either one). The beauty of this is you can get eurostar for as low as £70 an adult verses a few hundred per person on booking as part of a disney package.
Another advantage is that you only have to pay a low deposti to secure your holiday and you can spread the costs. Eurostar however must be paid up in full.
Lisa.
One thing to remember though is the disney express isn't part of the eurostar price, and you'll need to add these on the price of the package (£15 per adult, kids free). Though to be honest, if you are there for longer than a few days you don't really need to add this).