That's very interesting. I have a few old cookery books and most of them suggest using ready-made curry powder which was readily available in Victorian times. What is the book? The heat here is coming from mustard (Don't know if this is mustard powder or seeds) and pepper (I'm presuming whole white peppercorns but that's a guess!). I often use black peppercorns ground into curry pastes as my daughter doesn't like chilli. Even if using chilli, pepper adds another dimension of heat.