I used to make my own 'marmite' - you need flaked edible yeast which is a byproduct from brewing beer. Most health food shops will have it and some supermarkets as well. NOT brewer's yeast or breadmaker's yeast!!!! Flaked yeast - the edible variety.
Then mix in some soy sauce until it's a consistency that you want - maybe very thick or it can be runny. The taste also depends on the type of soy sauce you use - dark soy, light soy or mushroom soy. The latter is my choice. Vegemite is supposed to be the same but I can taste a difference and don't like it. Also, English Marmite is more salty than NZ/Aussie Marmite. But right now the Marmite factory in Christchurch is damaged after the Canterbury earthquake last year and you just can't get NZ Marmite in any of the shops. You can get a similar product called Promite, which is made in Australia and this is my favourite now.
Robyn