The MGB clutch slave is really easy to do - I did one in an on-street parking spot in front of an Italian resturaunt once because thats where it died (put the right side front wheel up on the curb to get a little room and did it in an hour or so). I know its basic, but check for fluid. If theres no fluid in the master, and no suspicious evidence of fluid under the master (inside or out), then I'd change the slave 1st. If you have fluid, then its not leaking and may just need a good bleeding. Complete replace or rebuild will certainly eliminate any doubts, but a simple bleed or slave swap could get you down the road quick if the master isn't failed.
I'm looking at the same problem on a 1984 Jeep XJ and it is NOT easy - the clutch slave is on top of the transmission and can only be accessed by dropping the trans (or cutting an access in thru the floor). I'd rather do a dozen MGB clutch slaves than one on that Jeep.