The rule on warranties is that you have to have the servicing done in accordance with the published schedule of the manufacturer, within the stipulated mileage/time scale, and the servicing agent should use manufacturers branded or directly equivalent parts. Assuming that is done, then all warranty work has to be honoured by the manufacturer and its dealer network.
There is a separate Forum header on recommended independent servicing garages.
I haven't used Motormech but used a similar operation near Nottingham, AlfaTechnico, for my servicing and couldn't have been more delighted with their work, at less than half the price of the dealership.
Also note that the oil change is NOT included in the standard Alfa service on JTD engines which have an oil sensor, and this could add another £100 to their bill and another day off the road when the warning light comes on telling you to get the work done.
My own inclination and advice, would be to find a good and reliable Alfa specialist who has the correct diagnostic equipment and understands what all the error codes mean and to ignore the big Internet companies.