Hello Roy,
The knob should pull straight off but you may need to take off the front plastic panel which will reveal a screw if there is one, expect it to be a bit tight when you pull it but don't force it, if you can't pull it straight off take the top off the machine first to reveal the screws for the front panel, take the soap drawer out also to reveal any hidden screws, usually one, absolutely essential that you take the plug out of the wall before you touch anything inside the machine.
Ken
drivewithken@blueyonder.co.uk, July 2007