Could be one of 3 reasons JayJeeves.
1) The slave cyl they fitted is faulty.
2) They did not top up the fluid resovoir after the job, so some quick cornering or hills etc could expose the clutch slave pipe outlet in the resovoir.
3) You also have a leak in your braking system, possibly a wheel cylinder so it is dropping the fluid, it is a kind of back handed saftey option the pipe for the clutch is higher than the brake system so when you lose fluid from your braking system the first to show up is the clutch.
You need to check.
Under the car are there any small puddles, also check around inside of wheel rims, are they wet? can see anything dripping from the bottom of the brake drums (guessing you have drum brakes) or around the calipers on the front (or rear if rear discs).
Fluid around the slave cyl if there is dry it clean it and drive it - then re-check it. to see if that was a NEW slave leak or remants of the old slave Cyl try and have a look at the back of it as well where the pipe enters there will be a bleed nipple as well, any fluid around them.
BE CAREFUL with brake fluid as it can damage paint work horrendiously and quickly and is also not good in your eyes either!