10lb ripe tomatoes sliced, 12 medium onions sliced, 2 1/2lb demerara or white sugar, 2 1/2 pints vinegar, 4oz whole peppercorns, 2 oz salt, 1 1/2 oz cloves and 5 tsp cayenne pepper
You can also add crushed or chopped fresh garlic or dried garlic granules or flakes, fresh or dried chillis and paprika if you wish.
Bung it all in a pan, stir and simmer for two hours until thick and soft, stirring occasionally. Seive and return puree to the pan.
Bring to the boil and simmer for 5 minutes. Pour into hot bottles and seal. Sterilize for 30 minutes in a water bath.
ETA: August 2015 - We've just finished off the very last bit of ketchup from the very last jar and it was still fine 4 and a half years later! It would have been finished off much sooner but one jar got forgotten about at the back of the shelf :-) It had matured from the rich red in the pan pic above, into a really deep dark brown rather like HP sauce.