I don't think there is necessarily a "best" combination . . . all depends on whether you want it nice to look at, or more of a barrier. If the former, just buy shrubs that can be kept at the height you want; having said that, Berberis, Escallonia, Cotoneaster and Copper Beech are all reliable.

Welcome to GoY!

Hornbeam will make splendid deciduous native hedging, as will beech and field maple. But I would never plant hazel as a hedge. It has very large leaves, too large for hedging, and it will never form in a dense enough habit for hedge. The leaves will be butter yellow in the autumn, at which time it will be attractive, but unattractive the rest of the year. The yellowest of all the native decidous choices is field maple. Whatever type of native hedge you opt for, all will be productive.

I assume you've bought enough plants for the length of hedge you want to plant Conx. I'd just mix them up randomly, spacing out the forsythia so they're not next to each other.

The website where I buy my hedging and trees says -
"we recommend planting 5 hedge plants per metre if you are planting two rows and 3 plants per metre for a single row hedge. You can use the same spacing for our Stock Friendly mix which is ideal for horses, cattle and sheep. The spacing between hedging rows should not be greater than 45cm if the hedge is to be stock-proof or laid.

So you've got enough plants for a 7 metre long hedge if you plant them in a single row, or 4.4 metres if you want a double row (thicker) hedge.

The most important thing will be to make sure the plants get REALLY well watered at least once a week - either by you or by rain - for the next year or so.

Thank you so much for your help and suggestions everyone. A little concerned re the comments about Hazel. The Hazel was recommended by my local garden centre. And unfortunatley I have already bought 10 small plants. They look very similar to the hornbeam in that the leaves are similar and you are quite right they are definitely quite large.

I do realise it is a little late in the spring season to put a new hedge in and I am watering the plants copiously. The chap who is demolishing the old fir tree hedge is quite a woodsman and is offering me advice and help along the way. He has advised me as above, water water water.

The old hedge gets demolished tomorrow - yippee!!!!

Once again many thanks for your replies it really was lovely to get a good response.

Thank you