The on/off switch is combined with the door release switch in one item - part number 154145 (a little over £20) and my problem was the door interlock bit had failed. The following worked for me :
Plug out, remove top panel, and open door + look up at switch panel - you see 5 slots under the switch panel (the 2 outer ones are mostly covered by the main front panel). Start at either side and work across, using as wide-bladed a screwdriver as will fit across the slot width : put a wood block under blade as a support and lever bottom of panel towards front - each has a plastic pin behind pointing towards the rear, held by a spring clip. Each will pop clear and when all are unclipped you rotate the panel outwards from the bottom edge, taking the timer knob and its drive linkage with it. To get switch out I removed the door and the screws around the main front panel - one is hidden behind a cover plug below the access port (to gain access to bottom screws on main panel, lever the bottom panel outwards along its top edge).
RonM, September 2008