A decellerometer can be used at any time at the testers decision. On the MOT inspection sheet it says only to use roller brake tester if it is a vehicle that the tester knows wont be damaged by the rollers I.E. Limited slip diff, 4x4 or fitted with traction control.
If the rollers pack up then the decellerometer can be used for every test but only for 48 hours.
I sorted my garage into 4 piles of stuff.
1. Stuff I`m keeping
2. Stuff I`m not getting rid of.
3. Stuff I might use..one day.
4. Stuff that might come in handy.