Do I need to refrigerate Nigella Lawson's Chocolate Guiness Cake (unfrosted at the moment)? Will be serving it on Sunday

If you keep it in an airtight container and wait to frost till Sunday, you should be fine. You could also consider freezing the cake now and taking it out in the morning on Sunday to defrost, then frosting it.

Keep it in an airtight tin or box, somewhere cool, but you don't need to keep it in the fridge, unless you live somewhere that's warm and toasty at the moment.... Room temp is fine