Welcome to Beoworld, Chas, and congratulations on your purchase! As the owner of two K's and one M, I can assure that the Beomaster 900 is a must have for every serious addict
Those old potentiometers can indeed get sticky. The knob is held on the shaft by that little screw; if you remove the knob, you can probably spray on the shaft without further dismantling of the unit and with a little luck, it'll start working just fine.
If you need to open the unit, you need to remove the two larger screws on the bottom, the two screws on the middle of the side edges on the back panel, and perhaps four small screws on the top and bottom edges (can't remember if the M had those). Then the chassis will just slide out of the rear (remember to unplug the internal antenna first). Various bits might fall off on the dial side, like tape/foam strips that once had glue, and the small light reflectors at the sides. Watch out for those, and put some new tape to secure the internal antenna wires in place before assembling it again.