"GGCN" is the Imternational Call Sign of the ship. Traditionally, British ships all had a four letter call sign beginning witha "G".
"C35" is the pennant number. If tyou travel up pompey harbour you will see all the ships alongside with big black numbers on the side i.e. "F239" and D78" etc. "C35" is (I think) Belfasts pennant number (i.e. the one that would be painted on the side).
Interesting stuff, thanks for all your contributions.
I didn't want to admit it in advance, but I was also wondering...
Unfortunately, I noticed the flags only while leaving the ship, otherwise I would have asked the RN guys (who were welcoming the guests onboard) to enlighten me.
VERY interesting tour btw, highly recommended.