callum88:
Is this a shared equity scheme offered by the builders or part of the governments homebuy scheme? The builders sometimes have arrangements with some companies and take a commision. You are right to research this. Also be careful employing the solicitors recommended by the builder as these sometimes side with the builders and don't properly represent your interest. This is advice that comes from other posters to this site who have had problems with other builders developments. I can't remember seeing anything adverse about Charles Church (persimmon) but you should check to see whether there have been problems, so you can try to avoid.
Do a google for Homebuy scheme mortgages, there are plenty of mortgage brokers that have freephone numbers, so you can ask the question. I suspect they would have a list of companies with mortgage products that may be suitable for your requirements. I would surprised if it was limited to 2 companies.
Huckster