If they're not fat, don't worry.
Hill ewes, and I suppose primitives too, should ideally be CS 2.5, downland / meat breeds CS 3 at lambing.
They say that we should let their condition go down a notch or a half a notch, so that we can bring it back up and have them at the right CS and 'on a rising plane of nutrition' for lambing (and same for tupping.)
They should have access to as much sugar as they want in the last 6 weeks, and in a form that doesn't take up much space as the lambs are starting to fill up the available room!
Generally any experienced single-carrying ewe on good forage shouldn't need cake unless her CS is too low. Too much cake could lead to an oversize lamb and a difficult lambing. So make the sugar available in the form of a molassed mineral lick (not a feed block), or sprinkle molasses on the hay.
Twin-bearers and of course triples will benefit from cake - there could be insufficient milk if they haven't had cake. My experience is up here in the far north of England - clearly the forage has far more feed value where SteveHants is!
This year we've lightly caked throughout tupping and will continue through now, as it has been such a horrible year for them and they've needed some extra help.