I'm pretty sure that if they were averaging mortgage repayments they don't take into account people who no longer have mortgages - ie 10 people, 2 have paid off mortgage, the average is based on the 8 remaining, otherwise it's not realistic.

Embassies/diplomatic residences are not included within the census figures. GG and PM should however, if they are staying there that night, alhough I'm thinking there's no mortgage on those properties!

Also, I can't remember if the question in the census is based on what you actually pay versus what the minimum repayment is - we pay more than twice our minimum payment so we get through it much faster, but if people just looked at how much our mortgage repayment was it would look like we had a massive mortgage, whereas it was just average (but now nice and small).