It depends what you mean by liable, I guess you mean that if you book the car in for a 36K service (which according to the Alfa schedule mandates a belt change for the TS) and then the belt goes at 45k and you find out it was never changed, could you go back to the garage and claim they caused the problem and claim for the damages?
Personally, I don't think so unless
in an itemised invoice the garage listed the belt as having been changed at 36K or you could prove that it was implied that this was part of the service to you, proof of this is always tricky if it was word of mouth of course. An invoice with "1x36K service" on it is unlikely to be enough for a claim as they could state that this is just literally a filters and fluids change according to nothing else but their own opinion. Similarly a stamp in the book in my recent experience counts for very little, in a recent claim I made against my warranty the invoices were regarded as the physical proof not the service book, I might as well have not had one.
I believe that the Sales of Goods Act would definitely apply if you had the itemised invoice and an independent expert then attested to the fact that the goods were not delivered. I feel it would be difficult otherwise.
The advice is always to insist on a fully itemised invoice for any work carried out and then put the somewhere safe, that way you have peace of mind and something legally binding to refer to if at some point in the future you discover something which was sold to you was not delivered.
Hope this helps