I'm sure there is a lower ratio did used in another IB5 box, for example the MK4 Fiestas 1.25 and 1.4 used a 4.27 diif (according to the Haynes manual) and also it depends on the wheel size. I sold a set of 12" classic Mini wheels to a guy using a MG Midget for autotests, because it would give him a lower ratio than 13" wheels and was far cheaper than purchasing a lower ratio diff.
Hi John thanks for that I did wonder if the 1.2 and 1.4 would have a lower ratio diff. Changing wheel size is not an option I am running 15 inch as you need big diameters for ease of rolling over obstacles which compounds the problem as in effect gears the car up.
I will investigate the other diffs thanks.