It's an income tax, so it would be related to earnings. The idea is to stop hammering people who are poor with unmanageable council tax demands, creating a system instead where people pay in accordance with their means.
So that is presumably people who bought their council flat for £2500 and it was valued at £100000 in 1990? Does that mean that when these people come to sell these places for £450000, the public get a cut of that or is the gain personal and the loss public?
Given that it's Scotland and they are notoriously stingy, my guess is that anything over the original purchase price falls to the Government. Did they not have massive over runs on the builidng of the parliament building?