Polytunnel - what do I need to know?

All,

What are your thoughts on building your own poly tunnel / green house, I was watching GW and saw a small cloche type plastic covering being put over a basic wood frame and they used plumbers piping as the support.

My idea was to use this on a larger scale to fit a space in my garden which would not house a ready to erect polytunnel primarilay because it is too narrow, but I thought it may save some money to put one together myself...

the space I have is about 1 metre by 8 metres!?

Thanks in advance