Mike
I've crossed Charles de Gaulle airport 3 times now (2004, 05 & 06) & it was quite easy - just follow the signs for the navettes.
Apparently it is even easier now with the introduction of the CDGVAL subway (T1>T3>T2) - 8 minute trip from Terminal 1 to Terminal 2.
From the CDG website.
http://www.aeroportsdeparis.fr/Adp/en-GB/Passagers/Arrivees/QuitterAeroport/EnCarEtTrain/To leave Paris-Charles de Gaulle airport using the SNCF French Railways station.
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In your arrivals terminal, take a look at the times of TGV, Thalys, Mainline and RER B regional express trains shown on the
information screens. These screens are located opposite the exits from the baggage claim areas in Terminal 2 (A, B, C, D, E and F).
To reach the SNCF French Railways station:
Terminal 1 : Take the Aéroports de Paris subway CDGVAL. Stop at "Terminal 2 Gare" and follow the signs for "Paris by train" or
stop at "Terminal 3 / Roissypôle" to use RERB to Paris.
Terminal 2 : Follow the signs for "Paris by train" (Paris par train) using the moving walkway.
Terminal 3 : Take the footpath (about a 250 meter walk) and follow the signs for "Paris by train" (Paris by train). Use RERB to
Paris or use the Aéroports de Paris subway CDGVAL and stop at "Terminal 2 Gare".
I use raileurope
http://www.raileurope.co.uk/ to buy my tickets. You can only book 3 months in advance.
Train times from CDG to LM (from raileurope)
d 1027 a 1207
d 1316 a 1450
d 1542 a 1720
d 2034 a 2224
Hope this is of some help.
Ade