Hi Seedling, as you know I too bought a Norfolk Greenhouse some years ago, before I knew better. I tried, with the help of several friends to build it and failed. It took a guy from our local garden centre, who builds greenhouses for a living, over 6 days to put up and at the end of it we couldn't sort out the window and had several bits left over. The semi regid UPVC which is marketed as shatterproof didn't shatter but broke away in chunks everytime there was a strong wind. I used to collect the bits from nearby gardens and stick them back in with greenhouse tape. After a year there was over 4 complete rolls of tape holding it up and all the metal had gone rusty. At this point I'd had enough and demolished it.
I bought an Acorn Greenhouse from Tebbs our local garden centre which was the simplest thing to put together and is still going. I have a second one which I built entirely myself in less than a day. The great thing is that if I have a problem I can pop in and see them and the owner came out to me to answer a construction problem. I'm pretty sure they do the sort you're after too and would recomend you try them first.