So, is a reasonable summary of this thread the following:
ptsb wants to make sure that you will be able to pay off the loan when the term ends.
If you can show them that it's in positive equity or that you have other assets, they will be happy and won't switch you to capital and interest.
If you are in negative equity and can't show them how you will pay off the loan, they will ask you to sell the property as the mortgage is not sustainable.
In either case, they could switch you to capital and interest under the contract, in which case you would either have to pay or go into arrears.
You could challenge their right to switch you to capital and interest.
Basis of challenging their right to switch you to capital and interest
While the contract is clear that they can do a review after 3 years, the illustration was for a one off payment at the end of term.