[Half a Mullard]

Success! I took Mike's advice with a slight variation. I was afraid of damaging the knob by prising the ring apart. However I tried another approach. I used a small screwdriver inserted in the slot under the ring and pushed upwards. Each ring flew off circlip style but was soon retrieved. As Mike suggested the knobs then lift off easily although ferrite rod brackets need loosening to give sufficient clearance. I will lubricate before refitting as suggested. Now destined for a good soak in wash liquid.

The various comments re WD40 make very interesting reading. I think perhaps my conclusion is to use sparingly and perhaps be very cautious with cherished items!

Thanks again. Will try applying power tomorrow.