"Will probably purchase them at a height of either 20-30cm"

Have you thought of taking your own cuttings . I used to do lots of laurel cuttings and they started out about 20 cm high

I would be very careful about planting a laurel hedge, if you plan to plant the common laurel, they can very quickly get to 8 metres high and 10 metres wide.
Between now and march is the time to plant them, provided the ground isn,t frozen, I would plant them about 4-6 feet apart initially, {you may have to thin them after a while} you will have to keep on top of the pruning to keep them within bounds, and they grow fairly quickly when they get established.
Personally I would think twice about a laurel hedge.

So would I. However if I was to plant one I'd plant them closer together... perhaps 2 feet apart... that way they form a hedge quicker and the competition between plants would slow their growth once they got bigger.

Imo there are better hedging plants than laurel( assuming we're talking about common laurel) One of the problems with common laurel is because it has quite large leaves it is not really suitable for trimming with a hedge trimmer as the leaves get damaged.

I rather like some of the evergreen viburnum's. Viburnum tinus is a favourite.It grows quickly buit not too quickly and has a long flowering season

http://www.flickr.com/photos/31559373@N00/3379402503/

Also have you considered a mixed hedge using a range of evergreen( or if privacy in the winter isn't an issue you could include some deciduous shrubs.

If you have a fair bit of room you could create a layered shrub border with a mix of ceciduous amd evergreen shrubs which if carefully chosen would suipply interest and flower almost all year around

Here are some examples

http://www.flickr.com/photos/31559373@N00/528130915/

http://www.flickr.com/photos/31559373@N00/3064885712/

http://www.flickr.com/photos/31559373@N00/2567629049/in/set-72057594072815906

http://www.flickr.com/photos/31559373@N00/3589247555/in/set-72057594072815906

Laurel is great if you want a large hedge and are prepared to maintain it. The smaller ones would be good to plant as long as they are well-rooted and sturdy. Plant 60cm apart, march would probably be ideal - as long as the ground is frost-free and not saturated. Prepare the soil - don't just shove them in subsoil because they are thuggish! Add compost if needed and water the bottom of the holes well, when planted, water in well and mulch to keep the moisture in. If you are in a dry area it might be worth putting a length of leaky hose in to water in case of a dry summer, this can be removed later. But in most areas you should be ok. When they've settled in nip the tops off to encourage branching. Make sure you prune them every year (late spring is good) as they will grow out of control very quickly without it. Be aware they exude a toxic gas from the leaves when pruned - harmful to some people if inhaled - so wear a mask. Kept pruned they make a handsome hedge of monumental proportion - and the flowers smell sublime!

Prunus lusiticana, the Portugal laurel, narrow, evergreen leaves with purple stems and white flowers would be my favourite. Buy 3 litre, pot-grown plants which will be about 2 foot tall with good roots. Plant them about 3 foot between centres and use Rootgrow which will help to get them established. A good, thick mulch and regular watering in the early years is needed. Whilst Viburnum tinus would be a good flowering, evergreen shrub, it will be a magnet for the Viburnum beetle so you would have to spray on a regular basis.

I'm intrigued - what plant do you mean when you say Laurel? Aucuba japonica? Or Prunus laurocerasus? It's just that one gets about 8 feet high, and the other 25 feet...

www.readyhedgeltd.com provide all sorts of varieties at all sorts of prices, heights, clipped, not clipped etc. Rootball normally starts selling in November, but they also do air pots and normal pots. Some things available only at certain times of year. Checking out this website could answer all your questions. I have used them and am more than happy to recommend for information, service, price and delivery. Happy hunting!

Following jimmytheone comment ... viburnum L. tinus ... I call this the moth eaten plant. Spraying twice a year, early spring and during summer, with regular shaving immediately after flowering died down, kept the beetles at bay. Nice for winter flowers and evergreen foliage if beetles kept on top of! Prunus laurocerasus (common laurel) has a beautiful white flower spike in late spring/early summer, so prune after flowering, unless you want the black berries, in which case prune in winter. You can also hard prune the base of the hedge and leave the tops to flower and berry - just an idea!!

I would be a little more sanguine about competition slowing the growth of the hedge, if it didn't also result in one out of every 3-4 plants dying four years later, making odd holes in that "green wall". I usually recommend planting them as far apart as 1/2 of the planned height--i.e., a 2 meter tall hedge would require plants about 1 meter apart. That gives the fastest fill in, without later holes.

I can't say i've ever had a problemn with die back when planting at 2 foot intervals.

Well, it may be different in the UK, or with the species of Laurel used there. : )

If you're in an area prone to drought I can understand your thinking but it is very rare for us to get long periods of drought in the Uk.

I've been a nurseryman over 20 years and a professional gardener for nearly 10 and I can't recall a single example of die back caused by drought in an established hedge.

We have drought--boy, do we have drought!--but drought is rarely a factor here, since we irrigate practically everything. On the other hand, most of my experience has been with plants with extremely aggressive root systems: Myrtle, Oleander, Glossy Privet, Texas Sage, Japanese Boxwood, etc.