how do i replace the start button on my hotpoint aquarius extra wma48?

Hi i have recently bought a washing machine second hand but found it to have some faults when turned on the door lock light will flash and i have to hold in the start button and it will start again but it does stop again and again throughout the cycle, but when i have wedged the start button in it works perfectly, i have replaced the door interlock but it hasnt made much difference, so i think it is the start button is making the problem. Can anyone help please?