baby died recently due to v.similar scenario i.e.exp.tank filling with hotter andhotter water until tank failed and flooded room below with boiling water-cause?-faulty thermostat-TURN OFF WHOLE SYSTEM NOW-ring for plumber
Sounds like it's pumping over. Boiler stat highly likely in which case you need a CORGI RGI not just a plumber. Could be a pipe blocked with debris and rust. In any case you should get it looked at.