There is a critical point here, which has perhaps been missed. When a couple or an individual seek to buy, they will make choices about what they can afford and, accordingly, where they can buy. In some situations, they will choose somewhere, as was said earlier, of lower value, because that is what they can access by way of mortgage; in other places they will go for somewhere of higher value but with a discount offered. They will make a choice about their purchase at that point. What they are essentially doing, when you put a second charge on a property, is making a trade-off between the discount—and the length of the discount—and the price they have to pay. That is what they are doing.
If you make it five years, anybody will go for that, whether or not they need it, as that will be the only way they can access the property. If you make it 20 years, it is much more likely to be the people who genuinely cannot access full market value who go for the property. This is a crucial point. If you really want to avoid deadweight squeezing out the people who might be able to purchase only with the discount, you have to have a longer taper than five years.