On mine, you take the front panel off and the button is up the top on the left hand side.
Hi
I realise this form is a bit old but I had the same problem this Christmas and it helped! However my boiler is fairly old and did not have a red button with an X but it was located on the back of the thermostat. My boiler is a Warmflow M5 and if you take the top panel off you get access to the circuit housing box, behind this there was a bolt but its hollow, it leads to the back of the thermostat! If you put something plastic in and push you will feel a very small click, this reset's the thermostat!
Took me a while to figure out, had to remove the thermostat to figure this out but hope this helps someone next Christmas too!