How can I identify the problem with my Neff U1661 AOGB02 circotherm oven?

Recently had a replacement element installed by an engineer to the above oven. However, it is still not operating correctly. Not sure if the switch has blown or if it is a problem with the thermostat. As soon as the main oven switch is on, whether for just the light or the fan oven option, the element heats up. Only when the switch is off does the element go off. The fan operates as normal when the fan oven option is selected.

As a replacement switch has been quoted at £150, we would like to ensure that the problem is with the switch before we go ahead. Any help would be appreciated.