Preheat the oven to Gas Mark 4, 180°C. Steam the cauliflower over a pan of boiling water for no more than 3 minutes
Melt the butter in a pan, add the flour and stir until the flour has fully absorbed the butter
Add the milk a little at a time, stirring thoroughly after each addition of milk to keep the sauce smooth. When all of the milk has been incorporated, add the mustard and stir to incorporate before adding the marmite and stirring again to fully incorporate it
Add 90% of the cheese to the sauce and stir until it has melted and you have a creamy smooth sauce.
Put the quartered cauliflower in to an oven proof dish and pour the sauce over the top. Place in the preheated oven for 20 minutes
After the cauliflower cheese has been in the oven for 20 minutes take it out, put the heat of the oven up to maximum, sprinkle the remaining cheese over the top and return it to the over.
After 10 minutes, remove from the oven and serve