The worst thing you can do with alpines is allow them to become waterlogged unless you are thinking of just using aquatic plants.It's a waste of time and money if you buy alpines and plant in gravel sitting on clay..
Thanks for the input and advice bendipa.
The idea is to have alpines on top, with enough gravel/grit/sand between the rocks so that the water drains between the rocks - I'll be careful to leave channels so that the water can drain away. Lower down at ground level I won't have alpines. I'll pick plants that can stand a bit of water. I have some seeds from an old variety of geranium which I think might work - it's quite invasive so I'm hoping that surrounding it with rocks should keep it in check.
I'd like the rockery to be quite tall, but I want to use materials I have available to me here. I don't want to spend money on it if possible, apart from on a few alpines. I figure the clay would be much like the impermeable bedrock of a real mountain. It would be like a mountain built to scale, with the wet valley below with water loving plants, and the alpines higher up.
I'll definitely be working amendments into the beds (for veg that needs to be well drained) that have lots of clay, but there is so much (the clay goes down as far as I've been able to dig, which is about 4 feet, and there is only 6 inches of top soil on top), that I'll have to buy a lot of sand and gravel, which I don't want to do, so I'll have to dig some out. I think it'll make sense to use what I dig out to build up the rockery to a good height - I'm thinking at least 4 feet high. I'll also be throwing in some big blocks of concrete that have been dumped on another part of our land that I want to tidy up... if I can figure out how to drag them over to the rockery!
Trillium - Thanks for the tips. Your expertise is always much appreciated!
BTW - the reason I keep mentioning gravel, is that we have a builders yard next door to us which has a big pile of of the stuff, and I'm hoping to get hold of some for cheap! Perhaps it might be better to try and get hold of grit rather than gravel though? I'll see if I can get my hands on bulk sharp sand too - I already have a few sacks, but I'm going to use them for potting up plants.