Some random thoughts:
"
we bought ready germinated plants of about 3 inches from local garden centre ... is about 7 feet tall"
Sounds like they are a conventional variety. I have always considered them a bit hit and miss because they need a pretty long growing season.
The varieties bred to suit the UK climate mature with a shorter season.
I expect the seed is all F1, and more expensive, and maybe Garden Centres don't use them to reduce their costs.
Of the varieties I have grown recently:
They have been short (4' to 5' max), and I suspect that is partly how they make them mature earlier.
They have been the "super sweet" varieties. Their taste is magnificent - all our visitors have been well impressed, and think I am some kind of Ace in the vegetable department; I haven't let on
Having said that, they are supposedly more difficult to germinate. They came coated in fungicide (keep out of reach of children, wash your hands
) and I germinated them on damp kitchen paper (pretty much 100% germination) and planted immediately the roots appeared (only a day or two) into Richy's newspaper pots.
They cannot be grown with other varieties nearby [including any near neighbours, as they are wind pollinated, so club together and plant the same variety
], as cross pollination affects the sweetness
Finally, which ever variety you are growing, have the pan boiling on the stove before you go to pick them, and hurry back. The sugar starts changing to starch the moment they are picked. Don't pick-for-the-fridge, nor for tomorrow
"
At present building a barbie so they can go straight on when they're picked,"
I have read that they should be put straight into a bucket of water as picked for, I think, 30 minutes, then cook with their cob's outer leaves etc. still on (dunno if you also wrap in tinfoil, that's how we used to do shop bought ones)