Scrub the lemons and wipe dry. Peel off the outer yellow skin, using a vegetable peeler or sharp knife, and cut the peelings into fine shreds. Tie these in a muslin bag. (N.B. If a marmalade with fewer shreds is preferred, add the shredded peel from only half the lemons.) Cook the shreds in 1½ pt. of water, in a covered pan, until tender. Meanwhile, roughly cut up the fruit and cook with the remaining water in a preserving-pan for 2 hr., with the lid on the pan. Pour off the liquid from the shreds, add it to the cooked fruit and strain it all through a scalded jelly bag. Pour the strained liquid into the rinsed preserving pan, simmer a little if it seems rather thin, then add the sugar and stir over a low heat till dissolved. Add the shreds and boil hard until setting point is reached.