Great thanks for all the responses.

Maybe I would be better off getting a blow away, especially if rot becomes a serious problem, I will do some further cost benefit analysis.

I think the main thing to decide on before continuing is the design/dimensions before dealing with ventilation and rot prevention.

As two posters have mentioned the height I have been giving this some thought, I don't want to upset my neighbour by going too high above the fence so what I could do is raise the height to 170cm, and dig down 30cm into the ground giving a highest point of 200cm.

To be honest I didn't envisage myself spending too much time in the greenhouse but I guess if I am going to go through the effort I might as well do it properly.

Photo of the plot attached, taken 20 mins ago. There is just lawn in front of the plot, It is West facing and gets sun approx 12:30 to 18:00