timolappalainen:
Then just poll the buffer like on NMEA2000 library and filter messages you are interested.
Humour me please: I'm still very much a noob and need some high level pointers 
Presumably the frames from the engine canbus are being put into buffers within an array as they arrive.
Presumably your NMEA code sends out its packets to the N2K network at preset intervals. Faster for some things like Engine Rapid PGN's I suppose and slower for Temperature PGN's.
Presumably I need to get your N2K code to look for the correct frame ID (e.g. 0x288) in the array and then extract the data I'm interested in (e.g. byte 1 for coolant temp).
How do I do this last part? Is it some form of If ID=0x288 then get the data from buf[2] or some such into a variable.
Your help, as ever, is appreciated.