Thanks, Doreen. It sounds like there's no way to avoid a bit of clumping. I added the liquid to the powder, a bit at a time, stirring furiously all the while, and it still clumped. Running it through the sieve got rid of the clumps, and the gravy turned out fine (especially after adding a couple tablespoons of red wine, yum!). As you say, not quite the texture or colour of flour gravy; the end result was more like a sauce thickened with cornstarch. But it was perfectly acceptable, not odd-looking or tasting at all.

Rachel