If you are a Marmite™ hater, leave it out but double the quantity of mustard. You may also, if you wish, add a little gravy browning to colour the sauce.
I found it tasted quite nice, but mine ended up looking more like casserole than a nut roast. To be fair I was in too much of a hurry to measure the ingredients accurately but I was close to the mark with them. - 06 Jan 2014