Pete Nicholson,
You are saying you want to increase later, but I'm not sure of your motives at the present time. Increase later is easy, whether or not they are 'accelerated' early in the year.
The criteria here is of how much brood there is in the brood box and how much space is available for brooding.
If the box is stuffed with stores, there is a simple way to increase the amount of brood - change out a frame of stores for a frame of foundation. Only when the box is getting full of brood do you need another box.
I shall quietly be adding frames in front of (and then, later, behind) the brood nest in the Dartington in my garden to extend the available laying area. The advantages I have, is having drawn comb available and no real need to disturb the developing broodnest. I dumped the puny deep National boxes in favour of 14 x 12s long ago. They now don't usually need fondant to be fed in the winter (which may lead to a congested brood box in spring).
My view of you 'Newbees with no drawn frames and wanting to go for it? It is a gambol' group is that you should learn to walk before you try to run and, no, it is not a playful stroll (gambol), it does need a bit of experience and ability to recognise the potential problems as, or before, they arise and be able to take appropriate steps to counter those risks. If you wish to gamble with your only colony in your first year, go ahead, but don't be surprised when you get in a bit of bother.
We already have the OP thinking there is a nectar flow in February? So plenty of scope to get in a bit of a muddle without even trying. Yes, Devon is a way off Lincolnshire, so maybe there is a nectar flow, but I doubt it.
A box under the brood would be more appropriate as the bees can build down into it, without compromising heat retention, and it could be moved on top later, when more appropriate.
You may be using terms without a good understanding (or you might not) but supers are generally for honey storage, but the same size box could be used for brooding, when I would not refer to it as a super. Supering often involves a queen excluder. Some (me included) fit supers but allow the queen to lay upstairs if she needs (no excluder). So you need to be careful how you use the terms, if you are meaning something different than the 'norm'.
RAB
) but I appreciate any advice and I know that a super is for honey. I was thinking brood and a half or full additional brood box. I obviously need to assess brood and stores but will wait until it warms up.
