Place the tamarillo, lemon, water and caster sugar in a large heavy-based saucepan and bring to the boil. Reduce the heat to low and simmer gently for 30 minutes.
Strain the mixture through a fine-mesh sieve or muslin into a bowl. Place in the refrigerator overnight, leaving it to drip slowly without pressing down so as not to cloud the syrup. (This base syrup is