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My scented-leaf geranium is very straggly - what's the best way to deal with it, or maybe it's meant to be loosely growing

It rather depends how old it is. Although I haven't grown scented leaf ones all the standard Regal Pelargoniums that I've grown are not particularly attractive after their first year.
They are easy to regenerate from cuttings though.

Yes, just pull off half a dozen good leaves and stick them in a pot of damp compost. Cover with a plastic bag to keep in the moisture and leave undisturbed for a week or so. You should get at least four new plants out of them.

I tried that once and they went mouldy - would this be the best time of the year to try it again or would it be better to root this plant in water only?

Now is the absolute best time to take soft wood cuttings, as spring is the time when they really want to grow.
By all means put them in water, but they will take longer to start growing roots.

To strike them in compost, push several around the edge of a pot full of compost (they seem to grow better when they are at the edge ... I have no idea why )
Put some sticks in the pot to support the polythene bag. Give the pot a good soaking with your watering can and allow to drain, then put the whole lot in a large polythene bag making sure the leaves don't touch the bag at all, and seal with a twisty tie.
Place in a propagator if you have one, or in a warm shady spot.
Leave for 2 weeks and then gently pull one of the cuttings and if it resists, it means it's started to grow roots, then you can remove the bag.

Having lots of extra plants would be wonderful as I've held back using the leaves in food and drinks for fear of weakening the plant