Thanks Chrome. I just went through Betfairs SP calculations and Chrome is correct. This does answer a lot of questions.

What my table did prove is that Betfair's CALCULATION to predict the final SP is very good - and hence why there was such little movement!

I will do some adjustments and base my snapshots on the last traded price. I will probably do 3 snapshots - at 120, 30 and 10 for a few different comparisons. This may now give me the results I was expecting!

I will keep you informed.

... Fred