Hello, I hope the following is of assistance.
As long as it is the seat belt minder that is the concern then this isn't a reason for rejection on the MOT. The only light that relates to the seat belt that can fail the MOT is the air bag light if there is a seat belt pretensioner fault.
You can confirm this at the official MOT manual website at http://www.transportoffice.gov.uk/crt/doitonline/bl/mottestingmanualsandguides/mottestingmanualsandguides.htm
Let me know if I can assist further.