Howdens/Lamona HJA3300 oven only works in "turbo" mode?

Hi,

I have a Howdens/Lamona HJA3300 electric oven and there is a "booster/turbo" setting to preheat the oven more quickly which you are then meant to switch back to standard before cooking.

The problem is if I switch the dial back to standard it just starts cooling down again. The element obviously seems to be working (as it heats up in the first place) but I just can't figure out what the problem might be.

The manual says not to cook in "turbo" mode so i'm not sure what to do.

Does anyone have any ideas what this might be?