On the http://uk.voyages-sncf.com/ site, I searched Paris CDG (we're actually staying near the airport) to London and it comes up with:

1. Aeroport CDG 2TGV to Marne La Val Ches on TGV Atlantique departing at 14:49 and arriving at 14:59

2. Marne La Val Ches on Eurostar to London St Pancras departing at 16:54 and arriving at 18:46.

Total is GBP 392.00 for all five of us.

Does this routing make sense? Looks like two hours of sitting around at a train station, but it's a big cost savings. Any problems you foresee here?