tumble dryer start button wont stay in when pressed?

My white knight 767c tumble dryer will run as long as someone is pushing on the button but as soon as you take your finger off it, it stops.
It first did this a couple of months ago but I was able to get it going sometimes by holding my finger on the start button and gently releasing it, however that isn't working this time.
Does anyone have any ideas what's going on here, and if so, is there anything I can do about it or is it just a "call a repair person" issue?
Thanks in advance for your help.