Zanussi oven temperatures fluctuating wildly. How easy to change thermostat?

I have a Zanussi dbl/oven - ZDQ495W - and the main oven temperatures have started to fluctuate wildly (by up to 20 & 30 degrees) . I'm assuming this means a new oven thermostat but can't find anywhere which will download a proper service manual for this oven.
I assume I'll have to take off one or more of the internal liner panels but which? Any suggestions very welcome.