When making a cake as a base line I usually halve the recommended sugar as I find it too sweet. Now I want to ice the cake for appearance and colour I notice that icing recipes are loaded with sugar. Is there a less sweet / lower sugar alternative that will allow me to decorate the cake
It's loaded with fat but less sweet: Cream cheese icing (example recipe) goes very well on some cakes (like a rich chocolate cake). It does however need to be refrigerated so is best for use on cakes that are to be eaten up when they're served. It can also be flavoured, e.g. with lemon/orange zest, with will further reduce the sweetness (and provide a nice contrast if the cake is too sweet for your tastes).
Another option is one of many variations on Cornflour buttercream (example). These use cornflour (UK)/cornstarch (US) and possibly flour, together with milk, to thicken a buttercream. A simpler version is just to replace a little of the sugar in buttercream with cornflour, but I haven't tested whether you can significantly reduce the sweetness without affecting the texture.
My favorite choice is an Italian meringue.
Italian meringue is whipped egg whites with still-hot simple syrup added. The addition of hot syrup cooks the egg whites, making it quite safe to eat without additional cooking.
The drawback is that it does not keep as well as a buttercream, and will require refrigeration if you're not going to eat it same day