Hotpoint First Edition 1000 intermittent start?

My Hotpoint First Edition 1000 has become a little unreliabe on starting. Irrespective of the cycle setting the machine has been taking progressively longer to start after the power switch has been depressed. It is now at the stage where more often than not it will not start without without a gentle slap on the front panel (discovered in frustration!). What could be the main inhibitors to it starting?