The ridge on top should face west-east, the door facing north or east. That maximises sun exposure assuming no shade. The prevalent wind is SW. Ideally the GH has some sort of shelter from winds.

Hi, welcome to GoY, it depends on the size of the greenhouse, if you have1 80ft x 30, then what Botanic says is correct, but in reality the vast majority of greenhouses in gardens tend to be 8 x 6 or the larger ones being about 10 or 12 ft x 8, so there is very little difference in size, so aspect is less important, Derek.

I never knew that Botanic, thanks for that. My ridge runs west to east, which I assume is what you meant? However, the door opening faces west. Is that a big problem, or should I relocate the door opening on the other (east facing) side?... Which shouldn't be a big problem, a few screws and a quick swap around with the whole ends.