Peel the pears, and cut into quarters.
Melt the sugar in the vinegar and any pear juice in a large heavy bottomed saucepan.
Add the pears and simmer gently with a lid tight fitting lid for three hours.
Remove the lid and simmer for a further three hours with the lid off.
Pot into sterilised jars and seal in a Bain Marie.
Test the seals when cold before labelling and storing.
The original recipe suggested pealing the pears, and leaving them whole, with the stems still attached. That's dangerous with my pears, because you can't tell if they have maggots or are brown inside until you cut them.
The first time I did this, I used 1kg of pears, and it produced about 750ml of the final product - not enough to fill my 1 litre jar!
The first batch use 500g of sugar, but that was too sweet. The second batch, I used just 300g, which tasted lovely on the day of cooking.