OK. I am here with a face stuffed full of humble pie. 
I have measured as per Sorrel's link, and I have read the explanation of cup size in relation to back size as per her link to that blog, which explains it all perfectly even though I don't think those two models look like their boobs are proportionally big enough in relation to their torsos to be D cups and I am apparently not the 36 E I currently wear but a 32/34 GG. 
The thing is, when I go bra shopping I do know better than to just go by one size, I try on all sorts, and if the band feels too tight then it's too tight, and if the cups feel too big they are too big. And at the moment I vary between a 34 E and a 36 E. Very occasionally an F cup fits better, but it depends on the bra.
So I'm none the wiser really. I do suffer from a lot of the symptoms listed in the list of 'signs of the wrong bra' but I'm fairly convinced I'm wearing pretty much the right size. I really can't see a 32 band being okay - I think it would cut off my blood supply and make fat squidge out everywhere. 
I will go into town and try on a 32 GG tomorrow and report back.