I need a polytunnel.

My plot at the moment is divided up into 4 large bed. The first bed as you go into the plot is basically clay/concrete. So today I have made the decision to put a tunnel on this bed and use the rest of the beds. The further down the plot we go the better the soil is.
As I have never used a tunnel before I am asking for advice on what to buy, where to buy. How much do they cost. I will probably be looking for a 4.5 meter tunnel.
Thanks in advance for any help.

I did exactly that Sunshine. Mine's 3m by 5m, wooden doors at each end. I didn't use thick enough water piping so I have had to add some extra wooden reinforcement to the frame. All in all I guess I paid around £140 on building mine, including the plastic sheeting which came in around the £70 mark via ebay. You can see on the pic below from a couple of weeks ago what I've done inside in terms of beds and planting.

Check ebay and local free papers for second hand ones. I have 2 both from ebay, 1 is 10m x 3m cost £90 plus around another £60 for timber frame and fixings - the cover was in good nick and its been up for over 4 years now. The other is 6m x 2.4m cost £50 but the cover wasnt too good so spent another £50 on a new one (from ebay) expect to get at least 10 years from it. It's definitely well worth keeping your eye out for these sort of bargains.