How can I fix my Carlton CSI 1S?

Machine is working ok apart from the dishwasher detergent tablet holder door does not release and so when the cycle is finished the door is still closed with the detergent tablet still in place.

I've tried the washer without a tablet in case the detergent cube was jamming the door release, but still the door didn't open. If I just throw the tablet in on the heat wash cycle it cleans everything ok, its just a pain to remember to do it at the right time.