Depends on your situation - how much grass/hay/haylage your horse gets and the quality of the same, and how much nutrition your horse extracts from it (ie, presumably as you're looking at the lite version, you've got a good doer!). I usually feed unmollassed sugar beet in addition to hay during the winter months - which were literally January to March last winter - as we have lots of grass.
The balancer gives the horse *all* its vitamin and mineral requirements, the lite version without extra calories being added, so you could upgrade to the regular topspec comprehensive or leisure time if you decided in winter that they did need more calories and wanted to add them in that way, or you could use sugar beet and/or a chaff to bulk the feed out. If you want to stay with the topspec range you could look at the fibreplus cubes for feeding alongside the balancer.