I've seen posts related to this but can't find them.
I'm currently on 512 premier and understood that to upgrade immediately to the max my line can support costs £14.99.
I went through the automated system and it told me the charge was £29.99, to change to premier 30Gb, which is what I want.
Have I misunderstood, or is the automated system wrong, and if so what do I do about it please, to get upgraded for only £14.99?
Were you trying to upgrade to exactly the equivalent product? The only reason I can see that would cause it to charge £29.99 is if you were trying to perform an immediate regrade AND downgrading your account. Take a look at this page and see if what you were trying to do could be classified as a downgrade.