This recipe has been surprisingly popular (I dont't like it much as its quite bitter but other people seem to like it!)  and is a way of using up left over lemon peel (see other recipes). Keep your used lemon skins in freezer after you have squeezed them and then defrost and make a batch. variatiosn include lemon&ginger and lemon&vodka. 

Lemon skins - weight them

Sugar - double the weight of lemons

1) boil the lemons in water - you basically need a pint for evey 1/2lb of lemons (yes, this does seem a lot!)

2) boil until the water volume has halved and the lemon peels are soft (about 30-45 mins)

3) Cool, then carefully take the lemon peel out of the water (I drain them in a sieve over the pan so no water/juice is wasted) and slice lemon peel (thick or thin, as you prefer)

4) return peel to pan, add sugar and heat gently, stiring to dissolve sugar - once dissolved, turn heat up to a rolling boil until setting point is reached (this will eb sooner than for regular orange marmalade as lemons have more pectin)

5) spoon into hot jars, seal and label

Variations:

Lemon and ginger (this is really good) - add abut 1/2 teaspoon dreid ginger to each 1lb lemon peel as you boil and then add diced crystalsed ginger right at the end. Check the ginger taste after you add sugar and add more if needed (DO NOT put hot marmalade in your mouth!!! cool before trying)

Lemon and vodka (popular with my more decedant friends!) - add a good slug of vodka right at the end, after you ahve removed from heat.