For my records, I should note that I got the crabby cider onto the next stage on Friday. The solid lumps of apple were removed and the liquid siphoned into a second, sanitised plastic vat. I had about a gallon of liquid so added 450g (or 1lb) of granulated sugar. I also took a small amount to put in a sample tube sealed over with clingfilm; the hydrometer floats in there and gives an easy way to monitor the progress of the fermentation.
The starting point was 1.034 at 18.5°C and the mixture has now dropped to about 1.030. This one might take a couple of weeks but I will watch for the point when the gravity stops dropping, a sign that the yeast has converted as much of the sugar as it can into alcohol and thus created an environment which is too high in alcohol to allow it to work any further. Then it will simply be a process of bottling (possibly through an extra stage of filtration to reduce the larger particulate matter) and storing for a few months to allow it to mature.