Mmmm, my mum used to make her own (Yep, gallons of the stuff). It was that tasty my mates at school used to buy my butties off me and I'd go the chippy!
As Tea said, make sure its proper colemans mustard powder, plenty of shallots and cauli. (we didn't put gerkins in but some people like it). I can almost smell how the house used to smell now!