AdderD, thank you very much, perfect explanation, just what I was looking for, I'm a whole lot clearer on the MAMx, and MIDx registers. The SAM manual appears to have errors in it.
Looking at table 41.9.15, Can Message Acceptance Mask Register (CAN_MAMx), the table below refers to MIDvA & MIDvB.... which is actually the table for 41.9.16, Can Message ID Register (CAN_MIDx).
Palliser, thanks for your help, I'm basically looking to use SAE J1939 as the PGN arrangement seems to offer everything I'm looking for to be able to transmit quite a bit of multiple sensor / input data in a single CAN message.
The following file may be of interest to someone : http://gurtam.com/files/ftp/CAN/j1939-71.pdf
Many thanks to both of you again,