Instruments and music:
Sheffield Music Shop sells string instruments and music
Allan King Pianos (Sheffield) also sells a wide range of string instruments
The Music Box, Meadowhall (Sheffield) sells music and accessories
DJM Music sell all sorts of music-related things by post
Electromusic (Doncaster) fantastic shop for band equipment
Dance of Delight  UK importers of Omnibows
Scayles Music (Edinburgh) great range of fiddle pick-ups, including MiSi
Stringmail  amazing selection of strings by post, great prices
Viola Ziessow if you want a really lovely hand-made instrument
Ebay if you want a bargain!

Music courses and examinations:
Associated Board of the Royal Schools of Music (ABRSM) exam board
Alexander Technique Offical Site great for musicians, back problems, balance…
University of Sheffield Music Department good courses, great ethnomusicology
Birmingham Conservatoire
Leeds College of Music
Royal Northern College of Music

Bands and sessions:
Big Eyes Family Players nu-folk and improv
East Handed old-folk(?) from Macedonia and new songs by us
Gypsy jazz session (Sheffield) marvellous music and beer