Cohabitation Agreement new Property England

hey all

I have recently purchased a property. 340k house, I put down 180k deposit, and have a 160k mortgage. I am the only name on the deeds, the mortgage in my name, it is all me from the paperwork perspective (I paid all stamp all legal costs everything).

I want to complete a cohabitation agreement for my partner who will be moving in with me. We been on and off, long distance, for 20 years, and have a 16 year old daughter who will also be moving in. My first question is, can I produce the cohab doc myself and have her sign it? (i can always get someone as a witness to sign), or should I be doing this through a solicitor? Some posts say solicitor, some say 'not required'.

The second question is on the cohabitation clauses.

I would like her to have a beneficial interest in the property by her paying 50% of mortgage payments. I would like her to build equity (which will be small, granted) by her 50% mortgage contributions.

For example, at the end of the mortgage tenure, she will have paid 80k of the 160k mortgage. 80k of 340 is 23%. I would like her to have 23% of the property, and myself have 77% (due to my deposit amount put down, and also paying 80k of the 160k mortgage).

It is very important to me that my initial 180k deposit is ring fenced. This is a non-negotiable red line for me. If anything puts this at jeopardy, then the only option would be to have a cohabit agreement where she agrees to rent from me (which would be at a lower price than the half mortgage amount so that it is beneficial for her).

The beneficial interest does get tricky if there is a breakup, and how to work out how much she has in equity. It is actually much simpler in paperwork if it is a rental agreement for a lower amount. And to be fair, she could actually save more this way if things were to turn sour.

I want to protect my future self by laying the ground work now.

Just to say again, I am the only one on the deeds, and the only one on the mortgage. I guess it's just if she has a turn in her head and wants to take court action for whatever reason? (i would never go back on any written cohab agreements)

Thanks all!