Hi Linda, as far as the width goes just let it grow out and clip it at the width you want; same with height, let it grow and determine the height you want by top clipping.
If too tall or wide prune it back with secateurs or loppers at almost any time.

Re clothing the base with foliage is another matter and while you are correct in saying that Ligustrum responds to renovation pruning, it is still going to be experimental as to what basal growth you do get from the stumps after cutting down. To induce basal shoots you would need to prune back as far as 30cm from the ground. Any higher would most likely just produce new growth towards the top and leave the base bare.
Perhaps it is a time to consider a new hedge or other boundary structure. Hedges such as Ligustrum, and particularly old ones take a tremendous amount of moisture and nutrients out of the ground to the detriment of anything growing nearby.

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I wouldn't replace a privet hedge if a privet is what you want. You won't kill it accidentally and chopping it near to ground level will give you a new one very quickly.

To keep the base green and fluffy you have to allow it to have light. Prune it so that the top is narrower than the bottom and the bottom will stay green. The green solid look comes from regular shearing at the size you want, but also from good branching. Don't be afraid to prune even when small. It will branch more and plants with established roots like these will still grow back very fast.

I feel inclined to side with Peter on this(even if Peter is from Yorkshire) if as you say the trunks are three inches in diameter, cutting to the floor will have its risks. That isn't to say that shrubs n bulbs isn't right in making the point that an existing hedge can grow back quicker than a new one. If it was an old hedge that had just been allowed to grow and was now fifteen foot high I would be happier cuting it right back, and I would add that the soil in Lancaster should work in favour of the hard cut option.

That brought a smile to my face Juno Mode. I sense a little tongue in cheek re the Yorks and Lancastrians. The fact is I'm a Cumbrian by birth, but what's in a county - just a different place to have ones home.

Perhaps in due course of time Linda will let us know the result of whatever she decides to do. One of the things I've observed over my years of gardening is that the cultivation of plants holds many tolerances and some very wide indeed. Only horticultural principles come near to certainties.

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Dear all

Thanks very much for all your advice. Still can't quite decide whether to cut it back or not!! I do want to keep it, so won't be grubbing it out. I think if I let it 'grow out' a bit it will get thicker - it's been pruned to within an inch of its life I think, but if this doesn't work, I'll try cutting it down to 30-40cm and keep my fingers crossed.

Love and compost
Linda

PS - will also feed and mulch it well.