In relation to the mortgage interest supplement, your figures appear to be accurate.
The other factors that would affect entitlement are
- what was the mortgage taken out for? If the mortgage wasn't taken out to purchase the principal residence (e.g. taken out to buy a holiday home, car, holiday, consolidate other loans), then it won't qualify for a supplement
- could you afford the mortgage at the time you took it out? If, for example, a person took out a mortgage while unemployed and could not afford it, it won't qualify either.
- are there any other people residing in the house who have an income (spouse, partner, adult children?). If so, their income will be means-tested as well.
- if the monthly interest is considered "reasonable" - this is a bit of a grey area, but many areas link it to the maximum rent limits which apply in the county you live in.