Its just an exercise in curiosity.
Essentially he wants to see if the interest paid over the mortgage has been eroded away by the increase in house price e.g.
- Paid £100,000 for house in 1995
- Use the calculator in his link to estimate what £100,000 is now equal to in today's money - £198,000 (rounded from that calculator)
- House is now "worth" £300,000 today (price if you were to sell it today)
- £300,000 less £198,000 (value of house currently less value of house in 1995 adjusted for inflation) = £102,000 "gained value"
- Total Paid in Deposit+Mortgage Payments = £250,000 (so £100,000 for house (point 1) and therefore £150,000 paid in interest)
- £150,000 interest paid less £102,000 "gained value" = £48,000 in interest paid (comparatively)
At least I think that's what he is trying to see.