Myson Boiler problems

Hi,

new to this and hoping for some help. i have an old Myson boiler that is a standard boiler with a tank for the hot water. it has recently started playing up, symptoms are, the boiler fires up as normal then runs for a short period of time (ranging from 10 seconds to 20mins) then just goes quiet and turns off, the piolet light remains on and the boiler is just about holding the house temperature above freezing.

can anyone suggest what the problem may be and any quick fixes?
coul dit be the thermostat? the water supply tank being frozen, the electric valve that splits betwen the water or heating???

please help.

cheers.