Hi Tarquin,
I stayed in Residencia Il Villino last June and was very happy with it.
Its a converted mansion house, with a small number of bedrooms. We stayed in a suite on the ground floor (since there were three of us). Its done up in traditional italian style - heavy mahogany furniture, but that suits the style of the building.
The owners are really nice and friendly. I cant remember the man's name, but he speaks fluent english and will do anything to help you enjoy your stay. There is a courtyard where you can take continental breakfast.
I had read on TripAdvisor that it can be noisy. I am a REALLY light sleeper, so was very worried about this, but with no need. Even though the bedroom was on the groud floor and looked out on to a road, there was no noise. The windows have 2 double glazes - i.e. 4 panes. Anyway, its a tiny narrow road, so there isnt much traffic (human or cars).
Its less than 10 mins walk to the Duomo. Some people on TripAdvisor complained that its far from the center, but I didnt feel that atall.
Its not a top class luxurious hotel atall, but its friendly, spacious, comfortable, and not too expensive.
In Sorrento, I cant recommend a hotel, but in Sep I stayed in a village 8km from Sorrento called "Santa Agatha nei dui monti" and stayed in a fabulous hotel! I nearly dont want to recommend it, since I want to keep it my secret! Its called "Oasi Olympia Relais". Its expensive (300E per night), but I asked for a reduction and they gave me 250E per night with no quibble. Its worth the money though - I'd go back in a flash. Its perched on the brow of a hill, and you can see the sea on both sides of the peninsula. I preferred being based in the village there, since there are hardly any tourists, and its a short drive to all the other towns (sorrento, positano etc).