I'm not convinced by the numbers either. The Halifax index says: "An average house, in the North West region valued at £450000 in 1992 Q1 would be worth £975230 in 2011 Q3. A change of 116.7 percent."
Or the other way round: "An average house, in the North West region valued at £525000 in 2011 Q3 would be worth £242250 in 1992 Q1. A change of -53.9 percent."
Both children had their own homes and were not resident in the family home at the time of its purchase, or since its purchase (With hindsight and capital gains in mind this was probably not the best course of action for the children)
I hope I'm not reading too much into that comment, but a response - even if the children had moved into the property for a time in the belief that such occupation would have secured a measure of private residence relief, they are unlikely to have succeeded. Temporary occupation does not amount to "residence".