Summer tyres – 2.3/2.4"
Winter tyres – 2.0"

Will this be horrible on the same rim – a Mavic 721?

Something to do with tyre profiles that I don't understand

That'll be fine on a 721. A narrower tyre will arguably be squarer and harder to lean over, but at the same time you can't lean over as hard in the wet anyway. In all likely hood you are using no where near the maximum cornering ability of your tyres anyway so I wouldn't worry about it.

I've used everything from 2.1 xc tyres – to silly 2.8s Michelin DH tyres on 721 rims.

I wouldn't worry about it. The only time it's going to be really a problem is if you rim is too narrow for the tire and that won't be an issue for the 721. The 2.0 is quite narrow for what is ostensibly a downhill rim but you'd have to a serious racer to notice the profile issue that might arise.
Out of interest how come you're using the 721? It is heavy enough for dh use and certainly overkill for trail riding unless you're super heavy and super clumsy, which I'm sure you're not!

http://www.mavic.com/mtb/products/ex-721.995015.2.aspx

Recommended tire widths: 2.30 to 3.00

Thanks both

Only using 721 rims as they don't seem that heavy unless compared to 717 which I've found to be not strong enough for me

My winter mud tyres aren't available any wider but it sounds like they will be fine

Thing with wheels, rims especially, is that you'll notice even 100g difference because it's rotating weight. Next time you're getting a rebuild consider either the stans flow or the mavic 819. They will save you over 100g per wheel and will be easily strong enough for bruising all mountain use even if you're a 100kg+ lump like me.