Mrs majee had the oil boiler on yesterday just to heat water for a bath for the kids. One of them flicked on the immersion switch too which she didn't know about. When we ran the bath the hot tap was spluttering vapour/steam (!) and eventually settled down after 5-6 mins to a steady flow of hot water. Does this suggest my cylinder thermostat is broken or just a consequence of both heating the clinder at the same time?