Thanks SSS,
I bought this bike, rode home and then started work on it. I know the tacho was behaving normally on my one and only ride.
I noted the Akropovic and when I had the carbies off doing the clearances noted the servo with cable hanging off it. I thought I'd do it all properly which is how I like to maintain my bikes. Then I researched - which has led me to here.
I've only ever got the one response with it off. After about 5 secs, tacho rises to around 7, drops to zero and repeats. I guess I will connect the servo and see. Obviously if it pulls the same trick I've got issues. I'll go do this and report back. If it seems fine with servo connected I will trial resistors as suggested.
Thanks for hanging in here.
Phillip