[gunner]

I'm planning on putting up a polytunnel this year, from what I've researched north south seems to be the prefered orientation, but due to the layout of our garden and buildings I can either fit a tunnel S.E-N.W or S.W-N.E, is either better than the other or won't it make much difference

[Aunt Sally]

Two Wests recommends that the long side should face South to get maximum sun, with the door at the West.  So l’d say, for you, it depends on what might cause shading on the tunnel - trees, buildings etc

https://www.twowests.co.uk/a/blog/best-orientation-for-a-greenhouse#:~:text=If%20you%20are%20planning%20a,it%20will%20capture%20more%20light.

[coldandwindy]

Put it so the prevailing wind comes across from the side, so it goes over the curve not slap into the flat end.

In most parts of UK, where worst winds are from the west, that means north-south orientation.