Hi Ben,
Thanks for the tips.
As it happens, I have done this trip now, and would recommend the following to any other travellors doing the same route:
- leave at least 2 hours for a transfer between CDG and Montparnasse (by taxi).
At 10am on the way out from CDG to Montparnasse, there was a traffic accident which meant that I arrived after 11am at Montparnasse, so I missed my connecting train (at 11am...ok, so maybe that was a bit optimistic orginally

). Fortunately I had booked a TGV Pro ticket, and was able to change at no cost to the next one at 12 noon, although I did have to go to a ticket booth to get it changed; for some reason the yellow ticket machines wouldn't accept the ticket to transfer, even though it said it should (and I tried the ticket several ways and several times :-0).
On arrival at Montparnasse I changed my return ticket to an hour earlier than orginally booked, just to ensure I had 2 hours between Montparnasse and getting to CDG (and that was actually an hour before the flight itself, to get checked in, security checked etc.).
Taxi cost either way was between 60 and 70 Euros. Not cheap, but probably quicker than waiting for the Air France Transfer bus and more convenient than taking Metro with luggage.
Regards,
-Wt