With due respect and without trying to cause offence but if you took on a 70% share and the Homebuy company took on the other 30% , at what point did you think the 30% would become yours without further investment?
While i do not know your full circumstances I would be pretty sure that this is not the fault of the Homebuy Scheme but of the adviser who dealt with you. The adviser should have given basic advice to explain how the process works. I am sure tht the Homebuy Scheme did nothing, have hidden nothing and continue to hide nothing about how their scheme works but if an adviser does not fully explain the process then you will not know yourself. If this adviser was a direct employee of Homebuy then I would challenge the advice or process he used. If the job was done correctly then it would be evidenced as such.
Don't forget that you can but the equity back at any time from the company (at current market value) and seeing as though the market has fallen in recent years it is likely that you will not pay any more than you originally paid for it. If you take this on a repayment mortgage then then the mortgage will be cleared within the 25 years. This option is ALWAYS going to be open to you, the only thing that could stop this would be affordability and credit....and if these things do get in the way then you would be unlikely to have taken on any more share than this in the first place.
Do also ask yourself the question of whether you could have bought this house if you hadn't had the assitance of Homebuy and also remember that if your 30% is worth approx £50k then if this means that your own share will be worth £100k and if in 25 years the Homebuy share is worth £100k then your own share on a repayment basis will leave you with £200k....still not ideal if you don't own the other 30% but a good start or deposit for a smaller house.
Get in contact with Homebuy and discuss the problem and ask them to evidence how this was sold and tio prove that you were given true advice and explanation and not just sold the product.
While you may have suffered at the hands of an inexperienced or an unscrupulous adviser thinking of his back pocket alone it does not mean that the Homebuy Scheme is in any way bad. It has helped many thousands of people get onto the housing ladder where they most likely would not have been able to. It has given many first time buyers the opportunity to then staircase onto larger proeprties where they may have struggled to gain their first. I am sure that you yourself felt very proud when you were able to buy your home, it is unfortunate that your case may be one of a very small number that may need recourse from either the broker who sold it to you or direct from Homebuy themselves.....If it was sold via a broker it is the broker you need to speak to regarding the advice and not Homebuy.
Good Luck