Add 175 g yellow mustard seeds and 175 g brown mustard seeds to 500 ml of a good hoppy local beer (I used Proper Job) and sit overnight in the fridge.
The next day add 160 g runny honey (around 8 Tbsp), 1 Tbsp sugar, 225 ml white wine vinegar, 2 pinches of crushed dried chilli , 1 heaped Tbsp of sea salt flakes, 1 tsp of (white of black) pepper and 1 Tbsp of mace.
Combine, put on the hob and heat to thicken. While you are doing so, use a stick blender, just for a few seconds, to break open maybe 20% of the seeds, this will help give a better consistency and release more of the mustardy flavours.
It will taste bitter but dont worry, it will mellow with time.
Put clean jars (8 x roughly 150ml) in the oven at 160c for 15 mins, remove, fill with the mustard while hot, put lids on, and leave in a cupboard for 2 weeks before use.
Adapted from the BBC