How do I reset my 9651 alarm?

I moved into a new house and the solicitor failed to give me the alarm code. After a year of constantly being on their backs they have just told me they are not able to get it, whether the previous owner has forgot it or just being awkward I don't know.
I don't have any codes for it, no manual or even an instruction booklet for it. It just has the date and time on the screen and there is a red light on it next to an exclamation mark inside a triangle.
Please can someone help me!!!!!!!!!!!!!!!!!