Here's the recipe, which comes from Shona Crawford-Poole's Ice Cream (Conran):
1 1/2 pounds of rhubarb (the recipe says spring, but it's well past the forced stuff now, so I compromised by using the thinnest stalks)
10 oz caster sugar (though I used less - about 9 oz I think, and about an oz crystallised out and stuck to the pan, so I think you could get away with 8oz if you like a more tart flavour, which I do)
Chop the rhubarb up, and put it and the sugar into a deep ovenproof dish. Didn't have one of these, though I suppose if I'd thought about it I could have used a casserole, but I used a 2" deep ovenproof pottery dish, and covered it with foil, and it was fine. Anyway, to get back to the recipe, you need to cover the rhubarb and sugar. It then goes into the oven (gas 4, 350 F, 180 C, baking oven of Aga) for 45 minutes. I did this in the Aga as its last gasp before I turned it off for the summer, and in that it took 35 minutes.
Once the rhubarb is very tender, let it cool, then either liquidise it or press it through a sieve. Being far too idle to faff about with a sieve, I liquidised it a lot.
Into the freezer then, and try and beat it at least once before it freezes completely. Having started this in the morning, I did manage to beat it. It was absolutely delicious, and, inspired by the hot weather we're having, and the fact I am very fond of sorbet, I am going to go through the freezer and dig out all the fruit we really should have eaten and turn it into sorbet.
Once the elderflowers are out, I am going to try combining those with this sorbet. Not entirely sure how I will do it, but think that I'll try putting them in the rhubarb/sugar mixture for the last 15 minutes of cooking and then take them out before liquidising. At least that's the theory at the moment!