I hope this belongs in Geographical, mods please move if it's in the wrong place.
We've discovered a lovely shop in the Harris Arcade (between Station Approach and Friar Street) called Adrienne Henry Millinery, which not only sells hats but is also full of vintage treasures (clothing, jewellery, cameras, militaria). Adrienne is delightful, happy to chat and if you're looking for something specific will try and source it through her contacts.
She also runs hat making workshops and parties, which I definitely intend to try out in the future, and has a website which is not yet fully up and running, but will be very soon:
www.adriennehenry.co.uk
