Aaargh. I'm going mad. Have an engineer booked for some work and want to change appointment. When I try and do this online via 'track my orders' I select the slot I want (showing available) and get a message back saying "can't be done online please call XYZ". I call XYZ and after wading through the nightmare of options get to the one that says, "to change an existing booking please press 3". When I press 3 I am told, "sorry, that option is not available".
So then I called the 'main' support number, went through a different set of menu options only to end up at the same failed "press 3" option.