Hi Jonny,
Do you know who manufactured the greenhouse for B & Q? E.g. Halls? If you could find out they may be able to help you.
At least you might be able to look at instructions for a similar model which would use the same fittings etc.
Failing that you need a vivid imagination, a degree in advanced mechanical engineering or a few very capable friends! ;)
Wishing you the best of luck!