AHM comes highly recommended, I've been going there fore years as have several other forum members, although it's based in Hartley Whitney (near hook) which is probably quite a drive for you ... but you do get a free courtsey car and top quality workmanship ...
I am also based in Southampton. I bought my GT from Black & White garage in Newbury, a 50 minute drive. I will probably get my next service done by them as they have offered me a dicsount because of a leak when I bought the car.
They are very friendly, and you get the impression they are more of an "old school" type garage with lots of experience - which I knida like - although I have read on the forum they can be pricey. That saide it did take them four attempts to sort the leak out, but I did get a courtesey car each time at the drop of a hat.
I would be interested to hear myself if there are anyother indies closer to home.
I've bought my last two Alfas from Black and White and used them regularly for servicing. Not the cheapest but their knowledge, dedication and courtesy put most franchised dealers to shame - though I've heard Meridian Milano (Portsmouth & Bournemouth) are good.