I have heard of the following as they are both in my local area, but cannot recommend either as I have never used them so I recommend further research in to them including pricing.
I've never used a Fiat specialist for any work, I have used standard garages whom I trust. I don't believe the engines are so complicated to require specialist knowledge for something such as a belt change.
Turin Motors, Sheepscar Road, Leeds
J B Motors in Morley, Leeds