It's always a good sign when a company publishes its response times but this appears to be absent on the globespan web site. I did find the following at
flyglobespan.com | Contact Us
All complaints must be made in writing within 30 days of your return to Customer Relations, The Globespan Group Plc, Colinton House, 10 West Mill Road, Edinburgh EH13 0NX, please note that no telephone communications will be entered into.
If you are contacting us please quote your booking reference number e.g.123456/SE7, lead passenger name, departure date (dd/mm/yy) and flight route, i.e. Glasgow / Palma. We cannot enter into any correspondence without this information. All communication must be in English and all replies will be in English.
you can safely ignore the constraints they attempt to impose, these could be considered a 'bad sign'.
So.
It's customary to allow 10 working days to reply to your letter. Probably worth sending another via recorded delivery (or even a certificate of posting will do, it will be deemed to be delivered no matter what they say to the contrary). If they don't reply in 14 days then it will be totally reasonable for you to take your complaint elsewhere.