Well, my grains arrived, so I put them in some full fat milk (as per instructions), covered with a cloth (I could have used a lid, but apparently the liquid is then slightly fizzy), and left them on the mantelpiece to do their thing. The next day, I strained out the grains and put them in more milk to make the next batch. The resulting liquid was slightly thicker than milk, with a pleasant, creamy, yogurty taste. I'm on day 3 now, and the liquid does seem to be a little thicker, so I think they're off to a good start. There's just enough for a small glassful (120ml or so), but as the grains grow and multiply I'll be able to increase the amount of milk.
I'm pleased with how easy it is to make the kefir, and I really like the taste. For a few pounds (£6.99 inc postage), I think it's well worth it, and it's much cheaper than buying pre-made (or in powder form).
AM