if i have 43 teeth on my crownwheel is it a 3.54 or a 4.11 diff ?

You need to count the pinion teeth as well to work out the ratio....

Divide the number of teeth on the crownwheel by the number of teeth on the pinion.

Accordign to my notes Rover Pinions have 10 teeth, apart from the 3.54:1 which has 13

So

4.7:1 47 Teeth on crownwheel Std LR & Rover 75 (from the '50's)
4.3:1 43 Rover 90 (may be different if overdrive fitted to car)
3.9:1 39 Rover 3 litre (do)
3.5:1 46 Rover 3.5 litre (do) Range Rover Classic

thanks guys, i'll have a count up