Toast the chillies in a frying pan over a high heat for a few minutes until fragrant. Place the chillies in a saucepan, add the garlic and 700ml/1¼ pint water. Bring to the boil and simmer for 15 minutes. Remove from the heat and allow to cool. (CAUTION: Cook in a well-ventilated area.)
Put the chillies, garlic and 250ml/9fl oz chilli cooking water into a food processor. Add the tomatoes, vinegar, cumin, allspice, oregano, salt and sugar and blend until completely smooth. If it is still coarse in texture, strain the sauce through a sieve.
Pour the sauce into a clean saucepan and cook for 5–7 minutes over a medium-high heat until it has reduced and thickened slightly.
Funnel into a sterilised glass bottle or jar and seal. It will keep for about 3 months in the fridge.