Funnily enough, I had exactly the same problem this afternoon🤷‍♀️ (same oven!). After studying the manual, I turned the oven off at the fuse box for approx. 5 mins, then turned it back on. Oven now working perfectly; 'STOP' message gone. Perhaps this'll work for you?
It may well have been a glitch that requires the oven to be reset, 🤔but try the above first (if you haven't already) before resorting to getting an engineer.
Best of luck! 🍀