If they cant read the BASIC instructions on the VOSA website why on earth would you let them anywhere near your car!?

And yes, the principle of a diff is to allow for a torque/speed difference. BUT the point of a LSD and ETC is to stop an excess of slip or "difference" - so testing a car in this way is a very bad idea.

More to the point why did they tell me I wouldnt be entitled to a free re-test if I took the car away, and why did I have to read the info off the vosa website for them to update them on acceptable process. On top of that my petrol light was on and the trip had been reset to 0!

Early classics and most disco's had a selectable locking diff - so this wouldn't have caused a problem with those. As the VOSA guidlines say - "vehicles with a limited slip diff should not be tested on a rolling road" 

Called into a garage yesterday and enquired how they MOT'd FFRRs with their rolling road, he said they programmed the tester that the vehicle was 4x4 and it only drove one roller at a time. Is that acceptable or is it definately a decelerometer only?
Mine's due soon!

de-accelerometer only, dont risk your range being damaged..

2-wheel rolling road = bad
4-wheel rolling road = good