Firstly could you give me the full ENr located on the rating label, its looks like -U1562B0GB/05 or similar
Copy and paste the following into your browser - http://www.neff.co.uk/store/sparepart/finder/detail/U1552N0GB/01 -- then spare parts list and exploded diagrams, then page 2. Come back to me when you have this in view.
The front knobs are only a push fit, take off the one that you feel is faulty and see if the shaft end is in anyway damaged.
OK sounds like your oven thermostat is faulty - if you take off the top and then follow the temperature knob backwards you should be able to see the thermostat shaft moving - can you see this happening?
No worries look forward to hearing from you.