I'm pm-ing you.

Pyracantha, hawthorn or holly are all good and will be excellent for wildlife - berries for the birds etc.
I think Leylandii is a terrible thug and you may regret planting it, so consider carefully before going for that option - just my honest opinion.

I agree with Weeding that Leylandii isn't a brilliant choice for hedges. How about using native species that are wildlife friendly? The traditional hedging plant is hawthorn and you can add other plants to it, such as blackthorn, wild cherry, dog rose and so on. The website I pm'd you about has native hedge plant mixtures for various uses.

Beech also makes an excellent hedge and keeps its leaves all winter, only losing them as spring arrives and the new leaves open.

Leylandii grows 3-5ft per year and can be a nightmare. It now has to be kept to 2 metres as a hedge which means an awful lot of work and rubbish to get rid of at least twice a year. They also sap all the moisture and goodness from the soil.
For a plain hedge Privet is good, but Beech is great and the colour is a lovely winter feature.

Many thanks for all your help and responses it is most appreciated. I now have a few options to consider as I don't want to plant something that I may regret. Our Site is fairly windy and exposed which probably doesn't help.

Plants suitable for hedging which survive well in cold and windy sites are: Berberis varieties, Prunus laurocerasus, as well as the hawthorn and beech already mentioned above.