I have now replaced the switch, which was an easy task. This is what
I did:
Switch off power.
To remove the top front fascia panel.
1. Unscrew two vertical screws (white with cross heads).
2. Use finger nails to ease up the rear edge of the fascia panel along
its entire length - it "sits" across the top of the fridge.
3. Carefully rotate the fascia panel so that it can rest on top of the
fridge (it has cables leading to it).
To remove the switch.
1. Remove the clip-on mains cable connectors
2. Use a small screwdriver to press in the small plasic clip at rear
of the switch and apply downward pressure to push the rear of the
switch into the slot.
3. Do the same at the front of the switch.
4. Push the switch down and then rotate it sideways so that the
terminals clear the hole.
Reverse the process to install the new switch.

This Bosch fridge freezer was purchased in 2000; I have seen other
Bosch models which look as though they have the same type of fitting
arrangement.

I obtained the switch from Inter-Spares UK Ltd of
Unit 4, Lincoln Rd Ind Estate, Leadenham, Lincs, LN5 0PE
http://www.inter-spares.co.uk

Keywords to aid searching:
KGV3120GB KGV3120 KGV 3120
Bosch switch part number 067747