Rub the quinces with a cloth to remove any fluff. Place in a large saucepan and cover with cold water. Bring to the boil, then simmer, uncovered, until very soft. The water will reduce
Strain the water through a piece of muslin (cheesecloth), squeezing out every last bit of juice, then compost the pulp. Weigh the quince juice and mix with the same weight of sugar in a saucepan.
Place over a medium heat and bring to the boil, stirring as you go (until the sugar has dissolved). Continue to boil until the jelly reaches setting point. To test this, place a small saucer in the freezer.
Once cold, dollop a bit of jam onto the saucer, wait a few moments, then run your finger through it. If it leaves a clean line with no jelly running into the middle, it's ready. The jelly will keep for up to a year in the fridge once opened.
For the cocktail, combine all the ingredients except the tonic water in a shaker with ice.
Shake, then strain into a highball glass. As always, gently top with your tonic, then finish with some ice.
Garnish with thyme and toast to Andy.