That is strange! I take it under the seat there is the mount for one?
Although it is obviously an MOT failure, in my 10 years of driving i have never seen a tester look for seatbelt buckles.... I guess they would just presume they were there! And im not talking backstreet ticket sellers, i mean very reputable indys aswell! Oh well i dont think it is possible to get a buckle and fit it onto the belt so it may have to be a full assembly