Economic Myths: Repayments of Principal and Interest

Banks are primarily balance sheet exercises. When they grant somebody a mortgage to buy land, they just 'split the zero' into an asset (the loan out) and a liability (the money which the vendor gets credited to his deposit account). This is about eighty per cent of all banks' assets and liablities.
The banks then have to show that the value of their assets (mortgage principal) exceeds the value of their liabilities (all the deposits). As we will see, this is actually irrelevant.
(The banks then make real profits by charging mortgage borrowers more than they pay depositors. So really they are just being paid to act as debt collection/risk pooling agents for people who have sold land.)
Most mortgages are based on the borrower paying a fixed total amount in interest and repayments of principal for the next twenty-plus years. Once a loan has been granted, this is all that really matters to the borrower and this is all the bank really cares about as well.
So for example, you borrow £100,000 for 4% fixed over 25 years, your monthly repayments are £533. You get statements at the end of each year showing that the amount of principal is going down, which is nice, but pretty irrelevant. What really matters is how many more years to go.
But you can work backwards from the £533. It doesn't actually make any difference whether that is a £100,000 loan at 4%, a £75,000 loan at 6.9% or a £50,000 loan at 12.1%.
Which is why I can't take the whole concept of negative equity and the corresponding 'threat tio the health of the UK banking sector' seriously. Psychologically, it can't be a nice position to be in, and it only really matters if you want to move home (apparently a lot of banks allow you to 'port' negative equity anyway).
So if you have a £100,000 mortgage @ 4% on a home 'worth' only £75,000, all the bank would have to do is write the mortgage down to £75,000 and increase the interest rate to 6.9%. That's you out of nequity.
Although the bank's balance sheet would look a bit weird (liabilities now exceed assets), it doesn't actually make any difference to anything in the real world.