clutch problems

My partner has a 2015 mk7 fiesta that has done 37000 miles, a couple of months ago on the odd occasion when she was slowing down to stop or trying to set off the car would shudder and stall, she would then put it into 1st, and even though the gear stick went into 1st the car stalls as though it hasn't engaged the clutch, or sometimes it wouldn't even go into 1st, only after a couple of attempts.

Over the next few months it became worse (but not constant) to the point where she felt it was to dangerous and could possibly cause an accident as it would suddenly stall setting off from junctions or traffic lights, so she took it to a garage, who fitted a new clutch kit.

A week after she got the car back the problem has arisen again but not as frequent, the garage who fitted the clutch took it for a good 4/5 mile run ( they also did this before fitting the new clutch kit) and the clutch was perfect and the machanic said he couldn't fault it, so has said he will not do any more work on it until he sees the problem personally as at the moment he said its just guess work as to what is causing it, the problem only seems to happen when the vehicle has stopped or when slowing down to stop, or setting off in 1st

I have checked the master cylinder and there is no leakage coming from it near the clutch pedal but was wondering whether we should have it replaced anyway

The garage said they couldn't find anything wrong with the old clutch when they removed it,and my partner has already spent £550 for the clutch replacement, then its an extra couple of hundred for the master cylinder to be replaced, so wouldn't be wanting to keep throwing money away replacing something that possibly doesn't need replacing.

I was wondering if anyone else has had the same problem and how they solved it, any help or ideas as to what it could be would be really appreciated
thanks in advance
Andy