Could possibly be the push rod seal?
I bought my car with a slippy clutch,found out it was just the seal.
Oil gets on the clutch and causes it to slip like mad.
Sounds strange how the gearbox could break whilst its in the garage?
If it got you there,and you have had no previous problems then I'm sure it's ok.
Get the car out and find someone capable of looking,new clutch and seal about £75.
I had a gearbox you could've had for nothing.unfortunatley I've scrapped it.

I'm pretty sure the mk2 box will fit,correct me if I'm wrong but you will need to buy a mk2 clutch also.