So if your circumstances changed and you had to move away from where you live now, and had to sell your property, if the property had doubled in value since you first agreed terms with the Local Authority, would you be happy to forfeit all of your profit to the L.A.?
You own the house and have funded (using earlier example) 2/3 of the cost. If your house had doubled in value, would you be happy to give the council a 6-fold increase in its investment as thanks for helping you out?
I'm not saying there should be open book when it comes to selling, but surely any rational review of this would suggest that having the L.A. and yourself splitting any profit according to your investment is fair, and the L.A. can then use its inflated investment to buy a similar share in another house for the next guy in the list.