After looking up various recipes, I decide on the following method:
Halve the crabapples and put in large pan. Cover with water, bring the boil and then simmer for about an hour.
Now the fun bit - you have to strain the mixture over night through a muslin cloth. Luckily, I have loads of muslin cloths. I bought a big pack of them when I was pregnant because they were on every list of baby essentials, but I never figured out what they were for, hence I still have a big pack of them. However, I don't have a stand to attach it to over the pan, so I rigged up a seive system:
Then, since recipes seemed to differ on the amount of sugar, I went for a three quarter ratio of sugar to juice. (1 kg of juice to 750g sugar). Add the juice of half a lemon and then boil on a rolling boil for about 20 minutes until setting point is reached:
Pour into sterilised jars and leave to cool.
To be honest, I think it was too much sugar as it seems quite a firm set, so next time I think I'd try a half sugar to juice ratio.