Do I need a solicitor to witness signatures for me and my husband agreeing that I get full deposit back before property equity is split?

When me and my husband bought our first house I paid the full deposit of just over £13,000 from savings that I had. There was a verbal agreement made between us that if we were ever to sell the house then I would receive the first £13,000 back before anything else would be split equally.

However, I now want this to be confirmed in writing just in case the unfortunate was to happen.

How do I go about with doing this? Do I just write a letter confirming the amount and what is to happen, get it witnessed by a proffesional person and signed by both of us?

Any help on this matter would be greatly appreciated.