The testers manual states
on retracting seat belts, check that with the mechanism fastened and the seat unoccupied, excess webbing is wound into the retracting unit.
failure if
a retracting mechanism does not retract the webbing sufficiently to remove all the slack from the belt with the locking mechanism fastened and the seat unoccupied.
Note: In doubtful cases, this should be checked with the seat base set in its rearmost position.
It doesn't appear to matter if it's slow as long as it retracts.
This is the MOT manual I have bookmarked -
http://www.motuk.co.uk/manual/contents.htm
