You would have to get hold of an oven thermostat to check the temperatures in the oven.
With manual thermostats there can be a +/- 10% discrepancy.
For example, set it to 170 and it could be as low as 153. (Most thermostats aren't this far out). 190 with a -10% discrepancy would give you 171 degrees.
If you can check the actual temperature in your oven against the setting you'll get an idea of how far out it is.
You would then have to call Beko to have them come in to rectify the problem.
This would normally involve changing the thermostat.
It is also worthwhile checking that the door is closing and sealing properly.
A sheet of A4 and the door closed onto it is a fair test.
Pull the sheet, there should be some resistance.
If no resistance then there's no sealing.
Adjust the door striker to pull it in a little.
Check it out, post here the result.
Hope this is of some help
Let me know how it goes or if you need more help