How do I override the control panel and turn off the central heating as the control panel will not allow me to turn off now the boiler is on.
This is a very difficult question to answer as I do not know which boiler type you have. Typically your control panel should allow you to shutdown the boiler, you should check the instruction manual for your control panel.
Failing that if you really need to shutoff the boiler then do the following:
Gas Boiler: Close off the gas supply to the boiler and switch off the electrical circuit supplying the ignitor. I
Electric Boiler: Switch off the supply circuit breaker at your home or apartment’s switchboard.
If in doubt get professional help!