Creda main oven?

Creda Europa S150E main oven fault.

Elderly parents called cooker engineer after I did a simple taks of fitting new element behind rear main oven panel this did not cure problem.
I noticed the 'ON' light below the switch did not come on so called engineer who confirmed the selector switch had gone.
They were told ' I will have to order new one' no mention of thermostat replacement.

They received a phone call today 9th apr
cost a total of £194.35 including vat when phoned back and complained it was high price for one switch replacement was then told we have to replace the thermostat as well that's faulty. this wasn't mentioned on the first visit, I was there, just replacement of the selector switch.
I think the complaining added the thermostat to justify the cost. If they had agreed at first I doubt he would have fitted a thermostat, just the switch.
Perhaps paranoid about elderly being stitched up but that's soundds it to me.

Q: How can a thermostat be tested without heat?

How hard is it to replace both anyway would it mean total removal of unit from housing or just pulled forward to access the top screws on the switch panel

Many thanks in anticipation.