In the UK, it is almost impossible to find celluclay in shops, so i have to buy it online. I have only ever used a similar product, which was very coarse and did not impress me. I've now decided to buy the real stuff, but i'm not sure how much i need. How far would a 5Ib or 12b bag go? And how much would you expect to pay for it, so that i know if i'm paying way over the odds for it?
Cheers

A 5lb bag will go a long way. I've had the same bag for a couple of years, 4 or 5 dioramas. I don't do very vertical dioramas, so my use is to duplicate flat surface covering.
I don't have the bag in front of me so I'm not sure of the actual weight, I would guess 5 lbs.
My point is - it lasts pretty well.

A pound costs under $10 here in the States so it should be about 5 pounds on your side of the pond. I use only a few grams for a 5X5 CM base for a figure. I have been using my current bag for at least a year and it's still more than 3/4 full.
When mixing it add cheap earth brown craft acrylic paint, some textured material and white glue (such as Elmers) with just enough water to get a very thick oatmeal consistency. Before applying it to a wooden base, drill a bunch of holes into the base to give the Celuclay more bite. Allow it to dry with no more heat than a 100 watt light bulb. It should be ready pretty much overnight. If it takes longer than that, it's either paid on too thick (keep the depth to no more than 10mm at the absolute most) or you've added too much water. Make adjustments next time. If youy need to build up your ground work, do so with packing or floral styrofoam. See my tutorial here.

I would add one thing to Al's advice, be sure to seal the wooden base with varnish or something. Even with just a little water the celuclay can cause the wood to warp or crack if it's not sealed prior to putting down the celuclay.

Hi there Luke:

I agree with all of the above...but think about this, before buying some stuff online try shreding some regular common white TOILET PAPER and then mix it with water, add some acrylic color and glue as iff you were working with Celluclay........no difference at all with results......

Celluclay isn't coarse. In the bag, the product resembles concentrated lint. It has a lot of drawbacks, in my experience. You can warp the base if the wood absorbs too much water from the Celluclay, and it takes a very long time to dry. The edges sometimes come up as it dries, lifting free from the base, but adding some white glue to the mix can reduce this problem. Celluclay is an organic material, and there are little crawling things that live by digesting organic materials, something I learned the hard way. I stopped using it when I discovered scores of weevils burrowing through a finished diorama base some years ago (I live in the subtropics).
Really, any material used for wall repair makes a decent ground cover material, and they all harden out much faster than Celluclay. I like Durham's Water Putty, a plasticized plaster type product sold in hardware stores (DIY to you Brits) in North America. It's a dry powder sold in a can, which is mixed to the consistency of batter and spread over the base. Then, dry Durham's can be sprinkled over the top, which absorbs excess water and creates an instant earth texture. The color is light tan rather than white, so it can be left in its natural color for some desert terrains. You can paint it when dry, or mix dry pigments with it before adding water.
Alternatively, epoxy putty can be used. It costs a bit more, but it's hard in two hours, and you can texture it, embed things in it, paint it, and drill it, if necessary.

thanks for the advice. I stopped using pollyfilla some time ago because unlike paper based products, anything you press into it (such as tank tracks) will become covered in sticky white paste which is hard to remove. I have been having quite a lot of success using a mix of pollyfilla and claycrete (rubbish celluclay) This doesn't warp or crack, but it can be hard to get the right earth texture which is why i'd like to try celluclay.