Marmite and cheese is a deliberate stack of two strong savoury things, and the craft is making salt-on-salt-plus-umami read as balanced rather than punishing. Marmite is a thick, near-black yeast extract that is almost pure glutamate and salt; mature Cheddar is itself sharp, salty, and savoury. Put them together without thought and the result is a wall of one flavour. The reason the pairing works is that the cheese, for all its own saltiness, brings fat and body that the extract has none of, and that fat tempers the Marmite's sharp edge while the extract pushes the cheese's savour deeper. The defining decision is therefore proportion: a thin scrape of Marmite under or against a thicker layer of cheese, the cheese leading and the extract amplifying it from behind rather than competing with it.
The craft is the ratio and the cheese choice. A firm, mature Cheddar cut or grated thick gives the fat and the structure to stand up to the extract, and it is the cheese doing the bulk of the work while the Marmite is held to the lightest possible film, often spread on the bread under the cheese so it never appears as a concentrated dark patch. Butter beneath bridges the bread to a salty filling and stops the Marmite reading as a smear; the bread is plain and soft so it carries rather than argues with two assertive components. The build is usually cold, but it takes well to being toasted or pressed, where the cheese melts down over the savoury base and the heat rounds the extract further, which is the form most often eaten as a quick hot snack.
The variations stay inside the savoury frame. The plain Marmite sandwich strips the cheese away and lives or dies on the scrape alone; Marmite and butter foregrounds the fat without the cheese; Marmite on toast runs the open-faced version; the wider British Cheddar shelf swaps in Stilton, Wensleydale, or Red Leicester against the same spread. Each deserves its own article rather than being crowded in here.