Boil the fettuccine in plenty of salted water, as per packet instructions.
As the pasta nears readiness, melt the butter in a small saucepan and incorporate the Marmite and a splash of the pasta water, stirring until fully dissolved.
Set aside ½ cup of pasta water. Then, drain the pasta and drizzle the Marmite mixture over, adding a bit of reserved pasta water if needed to blend. Serve with ample grated cheese.