The way it usually works is that at the point of job offer, you get 2 copies of the offer letter which outlines the key terms, both signed by the employer's representative (HR usually). There is a space at the bottom for you to sign one copy which you return to HR. You keep the second copy. I have never heard that witnesses are required for this.
But you've been there 9 years, and the original 'contract' will have been amended time and time again in that time. Not least of which will be thre annual letters one typically gets to change one's salary. Usually these set out the new salary, state that no other terms are impacted, and ask you to sign to say that you have received the copy.
Other parts of one's 'contract' are typically the company handbank plus a bunch of implied terms. These cover a multitude of things, but basically, if someone has worked like x for a reasonable period of time than it can be argued that x has become an 'implied term'.
All of this is kind of irrelevant to your situation because an employer can vary your terms provided it consults, treats you fairly (or rather not unfairly in the eyes of the law) and gives you notice.
So I suggest you don't try and use the lack of this document to 'prove' your point.