Who has the original? If you have it, take it to a local solicitor and get it certified as per NS&I's instructions. If the legal exec has it, ask him to get it certified by a solicitor in his firm.
Yes @Sirena I will do that, the original POA is with our solicitors. When I spoke to solicitors today they said often get the legal execs to certify POAs and have never had anyone question it. They are a large firm of local solicitors. It does look like NS&I are correct though. This POA had been accepted and registered with two banks, will that be OK, or should I send new POA when the solicitor has certified it?