A recent change suggests you have a slow leak under pressure. As you got the work done, I suggest you ring the changer of the master cylinder tomorrow and see what they say. It's not unknown for leaks to occur at joints as the ideal tightness is enough to stop leaks and not much more. You don't do brake nuts up to 'gibbon' tightness!
As for passing inspection, yes I've known a car with a very small leak pass a MOT.
