Toilet fixed this evening. It was the siphon. On its way out. Good old Dudley Duoflush. New siphon fitted. Thankfully the installing inside the unit looks pretty tidy. And there’s an inline isolation valve on the cold water inlet.
Nothing like a plumbing emergency in the morning to sharpen ones wits. Toilet cistern constantly filling. Ended up turning off at stop-cock until I can get someone out. Sigh.