Put the olives, onions, oil, green pepper and cauliflower in a saucepan and bring to a boil on a medium heat.
Simmer for 10 mins.
Add ketchup, mixed pickles and cooked mushrooms.
Return to a boil and then simmer for 10 more mins.
Stir often so that the mixture does not stick.
Remove from heat and chill for several hours.
Add a little lemon juice, if you would like it a bit sharper.
Add salt and pepper to taste.