Thanks for all your constuctive replies, I must apologize for the lack of information and the confusing way which I presented the data. I really didn't think the circuitry before the output was relevant as the circuit performs well without the snub diode. I can manually trigger interrupts and have good spark.
I did not think the value of the snub diode was that relevant. For all intents and purposes the value of the diode I had on hand should be close enough to try.

My reasoning is as follows:

I am only dealing with 1.2 amps on the primary even with a 1n4005 snub diode in place the coil should still charge, it won't like the reverse voltage but shouldn't prevent the coil from producing spark.

Thanks for all your replies.

James