STOP PRESS!
I've just found another way to do it. Once again, it's too late to get the really low fares but, if you can come back on Monday, instead of Sunday, this might suit you:
757 bus, at 1100 from Leeds bus station to Leeds/Bradford airport. (�2) Arrives at 1140.
1335 flight to Charles de Gaulle Airport. Arrives 1605.
(Total price for the return flight, including taxes & charges, based upon using the return flight as below is �137.40)
http://www.bmibaby.com/bmibaby/en/index.aspx
RER Line B to Gare du Nord. (A few pounds)
1745 train to Laon, arriving at 1933.
(�22 return, as before)
Returning, on Monday 30th, as follows:
1119 train from Laon to Gare du Nord. Arrives 1253.
RER Line B to CDG airport.
1635 flight to Leeds/Bradford. Arrives 1705
757 bus at 1825. Arrives at the bus station at 1900. (�2).
Depending upon exactly what the RER fare is, I estimate a total return fare from Leeds to Laon in the region of �170 to �175.
Chris