Mine has failled on this, as being outside the limits, does anyone know what the limits are from the SVA manual ?

Cheers

I think it can be up to 10% under

didn't read it myself but answer may be here?
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AFAIR it can't read under but can be up to 10% over.

It cannot read under so that no-one can claim they were sticking to the speed limit on their car and yet were still speeding.