I get the impression quite a few of the ticket office staff either don't know the policy or it's easier for them to ignore it.
Turns out it's the former.
To update, I decided to email the club to complain and within two days got a call back from them.
They admitted I'd been overcharged by more than a tenner, apologised and offered me £15 in gift vouchers as compensation to use either in the club shop or on future match tickets. They also confirmed the policy hasn't changed so that if a season ticket holder wants to move their seat they will just be charged 1/19th of the upgraded in season ticket prices.
So there we go, moaning works which must make some posters on here the most productive people on the planet!!!!