Mix everything together in a bowl. Cover and place somewhere cool. Stir every day for 4-5 days. If it needs thickening after this, add some mustard powder (about a teaspoon or so - it doesn't take much).

Bottle up and leave for a week or two to mature.

One of the beauties of the above is that whole white mustard seed is a natural preservative, so it seems to keep well (well the few months until the batch is gone!).

v2.0l.