Place the plum tomatoes, onion, garlic cloves, mustard seeds, sultanas, light muscovado sugar and white wine vinegar in a preserving pan or large, deep saucepan. Season well.
Place the pan over a medium heat and slowly bring the mixture to a simmer, stirring constantly until the sugar has dissolved.
Simmer for 1 hour, stirring occasionally, until soft. If the mixture is still runny after an hour, simmer for a further 10-15 minutes until it has thickened slightly.
Divide the hot chutney between warm sterilised jars. Cover with a waxed paper disc and plastic film lid or screw top lid. Set aside to cool.
Serve as a delicious accompaniment for bread and cheese or as a tangy side sauce at your summer barbecues. Keep in the fridge for up to 1 month.
Cook's tip: It is very important that you sterilise jars before storing any preserves. To do so, wash the jar thoroughly in warm soapy water then rinse. Being careful not to touch the inside of the jar, put it in an oven set at a moderate-low temperature to dry out and warm through for 5-10 minutes. Pour the warm chutney into the warm jar and seal.