Hello Martin.
Thank you for replying. I have now found a company called
'Steeltechsheds' they say their greenhouses are built to last a lifetime
and are able to withstand extreme weather conditions. The frame is made
of Steel Hollow section. If you could take a look at their website and
tell me what you think.

Having looked at that site, I wouldn't touch them.
The frame may last a life time what ever that means.
The plastic sheeting may last up to around 10 years but 7 would be more
realistic.It may well discolour with sunlight, and tends to get brittle.
David
Abacus Nurseries.
I can see no ventilation, and you should ideally have 1/8th of the roof
area as vents