Right then, next update....

I've started using an external clock source for the rpm after putting a scope on the car this morning and finding out a few more things.
This has been clocked to give me 26 pulses per second (based on a reading from a 1 second count command) and seeing as I now know for sure that I get 2 pulses per crank revolution (2:1 scaling on the crankshaft to the crank angle sensor on the back of the camshaft which pulses every 90 degrees on the cam) I'm guessing that should work out to about 780rpm.
I've chosen to go lower as it makes testing on the car easier as I can check at idle.

The pulses I get from a pulsin low and high measurement are...
1934 high and 1829 low (with a slight variation occasionally to 1935 high and 1838 low).

Running up the modified code (I've attached another version of the program as I've re-structured it somewhat to make debugging easier) I get a period value of around 9975 (fluctuates a bit though between 1966 and 1982).

Hope that make sense anyway

Leigh