Hi, as you have heavy soil, I would go for hornbeam, Carpinus betula, although both do better on well drained soil, the beech, Fagus sylvatica, prefers a light soil, and isn't suitable for very cold areas or frost pockets.
For a 40 metre hedge, if you go for a single hedge, you can plant either 9" apart, 175 plants, or 18" apart 87 plants, or you can go for a staggered double row, stock proof hedge, and plant them 18" between plants, and 15" between the rows, again 175 plants, either way 200 plants is too many for a 40 metre hedge, Derek.

Agree, Carpinus more attractive leaf and will do better than Beech in drought conditions (if it does stop raining!).

Hornbeam is better than beech in areas where its cold, shady or there's poor drainage - in sunny, freely draining areas, Beech is best.

Sorry - I had a phone call in the midst of doing this reply, but now I've completed and posted it, you've already had two other answers saying similar things!

Thanks guys. I'm in the West Midlands, so not particularly cold, but boy it was wet last winter!

All the hedge sellers seem to recommend 3-7 plants per metre, so I took an average of 5 per metre x 40 hence 200 plants. I planned to stagger them too.

I have another hedge question, which I'll post separately :)

Hi, yes they will recommend more plants than you actually need, you said it yourself, " hedge sellers" Derek.

Beech also retain many of their dead leaves through out the winter making it a little more interesting. However it does mean it sheds leaves all through the winter.