Hi there,
Are you jointly purchasing the property? If so you need to put a co-ownerhip agreement in place with your partner. There is no need for a declaration of trust/beneficial ownership IMO and a co-ownership agreement can deal with the broader issues.
You should ask your solicitor to prepare a standard document for you both to go through in the company of your solicitor, to amend it so that it matches your specific needs.
I strongly advise you to do this. You have no protection in the event that you do not put this agreement in place, and this website for example is full of posters for whom things have gone wrong, who have no protection, and who need to go to court to get anything done.