this just happened with me. i got the ha to down value from 280,000 to 250,000. at the moment they are desperate to sell so they do usually comprimise. they may ask to go to another lender. in my case barclays were the only lender so once i sent them the survey report it took them 5 days to amend
the thing is, where its shared ownership they are watched closely by the government so they must understand buying a property worth less than what you pay is not affordable home ownership
stick by your guns and dont be afraid to tell them you would prefer to loose your holding deposit than have neg equity, they will realise that no one in thier right mind will buy below what its worth so they wont want to loose a buyer thats already there, if they are repitable they should be fine within a 10% drop