Manual gearbox??
This has been noted on the last MOT, in which case it could be the tester covering his back.
Not a major problem unless it's really leaking out and leaving a big patch of oil on the floor.
Anyway, it's not a massive job to do, unbolt driveshaft from flange, remove driveshaft flange and replace driveshaft seal.
If its as good as you say, knock £100 off and get it sorted.