Hi there - welcome to AO!
Can offer a few suggestions based on my own experience only - I had a lot of work including cambelt, water pump, sump etc done by an independent garage (but not Alfa specialist) in Dunton Green, Sevenoaks called Antwis Engineering - I was very pleased with what they did and they sourced the parts they needed from Alfa where necessary.
I've used another independent in Redhill, Surrey called Service Plus who are excellent but not cheap...
and I've used Bianco Auto Developments in Hookwood (near Gatwick on the A217) who are an independent Alfa specialist to diagnose an airbag warning light issue which they did free of charge!
I've also used Alfa main dealer Allams of Epsom.
You may well find someone nearer you, but check some of these out as I've been pleased with their work. Good luck!