Place a pan of water on a high heat and bring to a boil.
Using a knife, make a cross in the tomato skins and place in the boiling water for 30 - 45 seconds.
Using a slotted spoon transfer the blanched tomatoes into a bowl of ice/cold water to stop the cooking process.
The tomato skins are now easy to peel off. Discard the skins and place the peeled tomatoes into a mixing bowl.
Add 1/2 cup of water to the tomatoes and gently stir to mix them together.
Add the sugar, white vinegar, onion powder, garlic powder, salt, celery salt, mustard powder, black pepper and stir thoroughly.
Cook the mixture on high, uncovered for about 10 hours until mixture is reduced by half and lovely and thick. Stir regularly.
Finally using a fine sieve, pour the ketchup mixture through it. This is to remove any missed tomato skins and seeds and squash any tomato lumps remaining. The result should be lovely and smooth. You can use a hand blender instead.
Transfer the now strained ketchup to a container.
Allow to cool completely before tasting. you can adjust the flavour by adding a little more salt, pepper or cayenne pepper.
When cool and tastes how you like it, decant into sterilised jars. I dunk a basil leaf into the sauce for a little added taste.