Your Hotpoint has an elecronic timer circuit which activates the lock. This is to ensure that the drum has stopped spinning thus avoiding the user getting a hand trapped in a revolving drum. Although this is annoying when we are in a hurry it does make sense since well meaning kids may open the machine door just to help mum or dad. This repair is strictly for the professional and may mean the replacement of the electronic module or the adjustment/replacement of the locking mechanism. Expect a bill of around 60 pounds.
Tony
May 2007