I am right in thinking that if I remove the super, there should be enough stores in the brood box to keep the bees going in the winter, as long as I check regularly and feed when needed
With a standard national, the brood box
can, *usually* (but definitely not in absolutely all circumstances for all bees - so not guaranteed) hold enough stores for overwintering.
For some bees, some sites and some winters it can be a bit marginal. Which is one reason that many prefer to provide a bigger brood stores space over winter. I myself prefer the deeper frames of the 14x12 to the aggravation of multiple brood boxes (double or brood and a half).
Feeding and checking.
You need to be checking
now to see how much stores there are in the brood box. And feeding heavy syrup to achieve something like 20kg/40lb stored before (I'd suggest) mid October or so - since you are in warm London. In colder places, the stores should be in place before the end of September. Last year, the bees were still working the Ivy through November down here on the coast.
Then you need to keep checking through the winter. "Hefting" the hive (feeling its weight) is a non-intrusive way of doing that -- you don't want to be opening the crownboard "for a look" during the winter. A see-through (polycarbonate) crownboard allows you to look without chilling! Great thing to have.
Start hefting the hive as soon as possible, to learn to correlate what you feel with what level of stores you can see.
If they run out of stores before spring, you'll have to feed fondant to keep them alive.