Gibraltar - Postal rates?

Does anyone know the exact express rate from Gibraltar to UK for 2003 ?
Or -- if not -- then the similar rate from UK to Gibraltar? A clue could
be 3,00 GBP, which was the price at the time of purchase directly from
Gibraltar Post. If possible, I would also like to know today's commercial
value of the stamp.

The question concerns specifically the NVI stamp issued by Gibraltar on 21st
June 2003 (SG 1151, Scott 941, Michel 1049). The stamp is unlisted in my SG
standard. It features Big Ben and a swift. I have the mint stamp and FDC
(and a buyer), who wants it for the express rate when issued.

Images of both items are available at
http://slaniastamps.heindorffhus.dk/frame-Gibraltar.htm
(Scroll to bottom of page).

Any help would be greatly appreciated :-)
Mette