opener knackered, control board fault or broken wire - possibly where door flexes.
Generic video here: https://www.youtube.com/watch?v=9XgL2WjmKWI
http://www.justanswer.com/uk-appliance/6bxds-dishwasher-powder-tabs-compartment-will-not-open-cycle.html#re.v/456/
http://www.fixya.com/support/t7045645-hotpoint_fdf780_dishwasher_detergent
XXX, January 2016