How to re-set my system from scratch?

I have a C&K Securitech, Securit 800L Control Panel. I have returned from overseas and moved back into my property which I LET for several years. I do not remember my user code or do not know if the tenants had one. I therefore have the system switched off from the mains as it will activate if switched on. Can you please tell me how to re-set the alarm, as if I was setting it for the very first time.