Don't worry about rain, but put it on a well-drained site if possible. Poke something through the plastic underground around the edges after it's all finished to make drainage holes.

Bigger is better, basically - less surface area for the volume inside. Gets warmer in winter.

I put wide mesh across one door permanently to keep out birds and cats, and hang loose mesh across the other. Slugs are bad - guttering across the doorway, set into the ground, filled with 'organic' slug pellets, and the legs of your propagating table stood in bowls of pellets. Put your chickens in there (if you have them) between crops to clear it out and convert slugs into eggs - an excellent trade.

Depending on what you're growing, open the doors by day and shut them by night is usual. Definitely, absolutely must open them every day in summer.

Easy-ish to DIY with a few big blokes. Is it £1600 just for the installation? Seems like a lot. 14' x 30' is a days work for two people to get up. I helped put up a 70' x 20' in a day once, but that was with two people who really knew what they were doing, and we'd already done a bit of the digging.
if you do put it up yourself, you need a warm sunny day to put the plastic on. It softens it and makes it possible to get it on good and tight. Chafe is the primary enemy of the polytunnel - if there's any movement of the plastic over the frame it will wear it away quite quickly, so you need the cover to go on good and tight. Use brooms to push the cover up and over the frame - makes it a lot easier. Set the bottoms of the doorframe poles a bit out from the end frame of the tunnel, so that the doors are sloped outwards-makes them open freely and stay open by themselves, and it makes the pleating of the cover at the ends a little bit easier. That's the hard bit, by the way.

I hope I'm not repeating myself here - I may have said all this before when this subject came up. That's a good big tunnel you're planning- you'll grow loads in there. I put two courgette plants in mine this year to see how they'd do. They went nuts - there's a marrow jungle in there now.

You buy this white tape that goes on the frame to protect it. It's the heat, too - the frames get very hot in the sun and soften the plastic til it rips.
The bigger ones seem generally more controllable for heat - no problems with too much heat, I don't think. Note edited message above with more tips... Small Daughter was climbing all over me earlier so I had to keep it terse.

Courgettes grow flowers of both sexes on each plant. Often the first few flowers are males, so they don't form courgettes. Once you've got both types of flower coming then you're in business. It can help to have plants close together. That's why you don't want to screen out insects - you need them to pollinate stuff.

I put them in the tunnel as an experiment - I had a bit of bed dug over and ready in there just when someone gave me a couple of seedlings, so I thought I'd put them in and see what happened. They don't struggle outdoors here, but they did even better in the tunnel. If you had the tunnel space, I think it would be worth growing them in there because the output per plant is that much better so there's less digging per courgette, if you see what I mean.

<blockquote><It seems I could pay between £2,000 and £6,000 for one. Is this usual? (was thinking of a 7 meter wide one by 14+ meters long. I might have to pay approx £1,600 if they come to install it. Everyone says how easy it is to DIY. Any pitfalls I should watch out for?</blockquote>

You can get a 14x30 for about £500, http://www.polytunnels.com/products.asp?isbaseunit=1 unless you are planning a top of the range professional jobby!

are the walls buried, or in the open air?

if they are buried, probably no need to insulate

a big roof like that is going to collect a lot of rain water, so good harvesting system for irrigation would be a plan.

wouldn't hurt, if you have piles of stuff sitting about.
i wouldn't bother if it involves distance or serious digging, but if there is a spoil heap kicking about.....

Thanks for the link stuartB. I did try this site, but I discovered that when all the 'added extras', ie, the essential bits were added on, the cost for a 14x60 ft long one was around £1,200. (about £1.45 per square foot)

I have found a site which can do me a 20x60 which seems to give me all I need for £1,643 - about £1.36 per square foot.

Looks like it will take 2-3 weeks for delivery, and, like everyone else, they will be putting up their prices soon, so I have been advised to buy a.s.a.p - but there again, they would say that wouldn't they

welshboy. Thats a great site. . . . . . makes me wonder what on earth I was thinking, buying an 'off the shelf' jobby. It came in the nick of time, I was going to order mine in the next day or so (from your second link as a matter of fact).

So glad to hear that I'm not the only 'tomato plant' - hence my reason for wanting a polytunnel, but I assure you, our 'derelect' green house area is pleasant to work in for most of the year - and when the weather is really bad I consider the area is 'dormant' any way.

Would the silage dump have been orientated to make the most of solar energy in the first place - so be warmer? would you be better off having a polytunnel of your own somewhere else, then transferring crops to your open 'silage' area during the growing season?