Hi I have a kia sedona and for the last 9 months or so the front heater does not turn off or down. You can still change the temperature setting but it is on full blast.
I do not know what a '56 plate is' (I know it is a UK thing..............) if it was built between 2006 to current I would be looking at a blown fuse particularly the ROOM FUSE, it has happened to me and come up as question as well.
The ROOM FUSE fixed this one for me (under the steering wheel). I didn't have a 7.5 amp fuse, but I put a 5 in and it's getting me by for the moment.
Note -- the burned out Room Fuse also threw an airbag light on, put the rear AC fan on full blast, and turned off the interior lights.
I had the instrument panel in the ceiling break apart when I pressed a slider door button in it and the fuse blew (likely the electronic board above it made contact with the metal bracket). Scared the heck out of me -- panel fell to the floor, fans front and back went into full blast, air bag light came on . . . the car seemed completely out of control. Replacing the fuse and having most of the car come back to normal was a relief, and I thank you bopbo for the answer.