My plan had been to use the fondant as their sole food supply (and any stores in the brood chamber). I have a bakers who is going to order me a 12.5kg block and I have two hives so is this enough? I plan to use OA in the winter so putting it on a queen exclude sounds like a must.
In my experience the 6.25ish kg is more than enough for one colony along with whatever is in the brood box, but somebody will come along and tell you it isn't enough.
The danger time isn't usually the middle of winter, it's if there's no natural nectar available in the spring or if the weather is too bad for them to forage.
I know people who make their own fondant, but time taken + use of cooker probably makes it cost about the same as buying it from a local baker. There's also the risk of overheating it, which turns it into an unusable waste of money!
Fondant is fiendishly sticky stuff. Wrap the top and sides with either plastic or foil. You can wrap across the base too, leaving a slit rather like with a tissue box. Put the slit at 90 degrees to the top bars, so there's maximum access.
the obvious advantages of fondant alone is for those with distant out apiaries that they wish to shut up and leave alone.
Not just distance, but weather can make checking colonies a bit difficult even if they're fairly close to home.
If there's a heather crop it may get too cold too soon afterwards for syrup feeding to be a viable option, and fondant is less work for the bees because they can eat it without needing to process it further.
Another benefit is that fondant doesn't go mouldy or ferment, which syrup can do if the proportions are wrong, although it can dry out if it isn't properly wrapped. (plastic containers will do)
There's this page about feeding fondant, where the writer gives a whole block per colony. No mention of leftovers in the Spring
http://www.stratfordbeekeepers.org.uk/PENotes/Fondant.htm