Are you getting married in the short, medium or long term?
The issue is associated with the period after you draw down the mortgage and before you're married.
If one of you dies then your mortgage protection policy should kick in and the remaining person gets the cash value of the remaining mortgage. Pardon my frankness but if you die then its less of a problem! However if your fiance dies and hasn't left the site to you in his will then it gets complicated. A legal mind would have to take on this discussion from here if you want to know further as I'm out of my depth!
If you split up or one of you decides you want out then you'd probably end up having to repay the outstanding mortgage at that time and negotiating an acceptable compromise. While its a possibility to consider it'd be the same situation if you were married and splitting i.e. negotiation. The alternative is a pre-nuptial type contract effective immediatially and if you're getting into that over a site, well good luck to you. My personal, unprofessional advice is don't sweat the small stuff and keep an eye on the big picture. Hopefully someone else can give you a legal perspective on the various forms but I'd say sign and get on with it as there's a lot of legal scary sounding mumbo jumbo which is basically just common sense. Worst case scenario and no-one on this site makes you any the wiser, the solicitors you already have should assist.