We're selling a shared ownership home – what is a fair price?

The housing association’s surveyor seems to have priced about £50,000 too lowQ We are currently in the process of selling our 40% share in a shared ownership property.The rules of the housing association are that we have to sell for the price a Rics surveyor sets.
We had our valuation done last week by a local, recommended chartered surveyor and it came in at about £50,000 less than what we were expecting. Now I know everyone thinks their house is worth more than it is, but we