Guys, thank you so much for all your replies and tips, the garage is an independent one in Blackpool on Commons Rd, I have taken all my cars there for the last 10 years I've lived here in Ireland and yes, the gearbox is second hand but he did not tell me how many miles it has on only that all the parts have warranty should anything go wrong in a few months (hopefully not).
Car is a 147 1.6lt. and has 69K miles on the clock.
He told me that a new gearbox would be at least 500€ not including labour.
Dermo and Red156Ti many thanks for your advice on giving Garage Lee a call, I simply assumed (obviously I'm wrong) that a dealer will almost always def try and rip you off but it doesn't seem the case for them, good to hear!
Red156Ti you are saying the slave cylinder could be the one leaking fluid?
This mechanic has apparently a lot of experience with Alfa cars and he was telling me that very few garages would actually touch an Alfa (yes it is a rare breed but we're proud of it, am I right?)
Also my father in Italy who has refurbished few vintage cars and knows them well is saying he does not understand how my garage says to change the whole gearbox if they're not even sure where the problem is coming from yet...!