Carshalton Choral Society
The music for HMS Pinafore is not available on the John Fletcher web site. It is available for free elsewhere and here are some suggestions.
If you prefer to download it complete as a MIDI file then you can get it at http://www.gsarchive.net/pinafore/web_opera/operhome.html
Alternatively you can play it directly at http://www.cyberbass.com/Major_Works/Gilbert&Sullivan/gilbert&sullivan_pinafore . . .
You need to have logged in successfully with the appropriate subscription to access these files: a Free one if they are out-of-copyright; a paid-for one if they are in-copyright