So, I've used TuneIt! and one of my SingStar mics today and tried to set the tension of the belt. What I found is that the mic registers pretty OK. However, taking the measurement changes the actual belt tension. So after hitting the belt with a spanner the first time after turning the engine to 30 degrees BTDC , the tension in the belt changes (Frequency drops from 114.5Hz to approx. 113 Hz). So for comparison, I always turned the crankshaft 2 revs ( to be sure) and then performed the same measurement. After a bit of such fiddling, I got a pretty consistent measurement of around 114Hz, provided this was the first measurement.
I have a Krikit on order from the US, so will check the tension with that once I have it.
Thanks for all your help
Dennis