(T) bump noise when changing gear

Hi just had new clutch but now every time we change gear we get a clunk noise,the garage says its probably movement of worn engine mountings,but it was ok untill they changed the clutch.
Any Ideas do you think they have forgot to tighten something,is it safe to drive,Im a bit of a dodo when it comes to cars as Manuel from Fawlty Towers would say "I know nothing"

gearbox mount needs to be checked and also all 3 gear selector rods where they attach to the gearbox. they're hard to tighten so one or more may be loose or threaded, or the rubber bush in the mount may have been damaged on removal.

also i'd check the fluid level in the gearbox, they may have neglected to fill it sufficiently

Hi
How bad was the clutch before? You may not have been putting enough stress on the engine before to move it.
You also have to adjust your driving style slightly with a new clutch as it will be putting load on the gears quicker and you may be beating the synchro.
Can you feel the clunk on the gear stick, steering wheel or seat as well as hearing it?