To be fair to Alpkit, they do explain their problems here
http://www.alpkit.com/colab/notes/pipedream-nightmares - and as they put it,
Because we develop our products and purchase direct from the factory, our lead times for new production are not always as quick as we would like. We also try and ship our products by sea whenever possible. This is obviously much slower than air but helps keep the cost of the products lower and does less damage to the planet.
Boats are slow, and a watched boat never docks.. so we sweat it out. We prefer not to take advanced orders, shipping schedules can change, last minute production runs get delayed.. in short there are many factors we cannot control and so we do not want to have your money stuffed down the back of our sofa.
What we do is make a note of any enquiries, it is as good as reserving the product, and we will let you know as soon as we have them back on our shelves. We also publish the expected delivery date on our site, however for the reasons explained above this is subject to change at anytime.
Frankly, I'm pretty sure their not happy either, and this situation has come up before, since a quick look on the net has someone asking about the waiting list in 2010. A year or two back they posted on Outdoor Magic about stock problems, and this is what they said. They, quite rightly, don't want to over order and end up with cash flow problems, but are of course very popular. Apparently the Pipedeam 400 is one of their best sellers, so supply is always going to be a bit of a problem.
Because of their long lead times and their relative size in the market, they don't get to order extra stuff from the Far East instantly, unlike a bigger brand. The latest example of this is the Rig 7 tarp. After a cracking review from Trail magazine (best value and overall), they sold like hot cakes. They managed to get the factory to run up more pretty quickly, but in different colours (the original kelp and black ones sold out in less than an hour!).
I can certainly understand your frustration (I wanted some dry bags a while back - and yep, they were out of stock), but how about something else? OM had some discussions about the 400, and some people recommended Cumulus and ME Helium's - see what you think
http://www.downbags.co.uk/cumulus-ultralight-350.html They are a bit more pricey though. The other possibility is PHD's sale. It should be coming up soon (its normally in September), and perhaps one of their Minim 400's would do fine. £250 normally, but in the sale? Have a quick look at OM's bargain thread or just signup for their email service. As for the bags, have a look at this
http://www.petesy.co.uk/phd-factory-visit/ - its like your bag was made by Savile Row!