the new goverment help to buy scheme is for any one (you dont have to be a first time buyer) but it is limited to new build (i think have to be less then 3yrs old) and limited to houses under £400k.
basically the goverment will be a guarantor on the top 20% of the property, and you can then get a mortgage for the remaining 75%...needing a 5% depoist.
so for £100k house you need a mortgage for 75k, 5k depoist and they will guranted the top £20k. not sure hoe it works once ur in, like if ur given the opportunity to purchace the 20% as time goes on.
if u own a house that in neg equity you would have to sell and pay the excess out of ur own pocket. or you could use the new scheme to buy a house and remortgage ur current house as a buy to let and rent it out.