With an interest only mortgage you repay only the interest on the loan. The capital remains unpaid until the end of the mortgage, unless overpayments are made, at which time the lender will be requesting the full amount borrowed.
An investment vehicle (like an ISA, Pension or Endowment) would usually run alongside your Interest only mortgage to ensure that the capital is repaid at the end of the term. This is not required by all lenders but it is your responsibility to ensure the loan can be fully repaid when necessary.
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