it sounds like you need to have a mini expansion vessel fitted on your mains supply as close to the boiler as is possible.lit is probably firing when you turn the tap off rather than on and as the water " bounces" within the pipework it will lift a flow switch causing the boiler to fire
if you look under the boiler there is a hole about the size of a ten pence piece. if you put your finger inside it there is a reset button which may bring your boiler back on. beware though as this is an over heat thermostat so something has made this trip. check the pump is circulating and...