Is it the reverse light switch then? I would like to say that it will pass the MOT without them working, so i wouldn't worry TOO much!! I only really use mine for seeing behind when its dark, oh, and to bee seen!! If mine did stop working i could just put my rear fogs on, they're quie bright (i rewird it to be both sides).
Anyway, i digress, see if it is the switch. It tells you about it in the haynes manual. I would almost bet it was that.