What are the implications when a mortgage compant fails to issue a default notice (scotland)

Hypotheticals are never easy to answer.
The lender can claim that they have issued anything they want, and that the recipient did receive the notices. Or that an address changed, etc.
A court will not look kindly on the person who defaulted.
Please tell us details.

I would talk to a real estate barrister in Scotland.

As Joy said, more info is needed. But basically, if this comes to court, and you claim you didn't get a notice, the lender will claim it was sent. Plus the fact that you would know whether you paid, it is unlikely to have much impact on a any court case.

You know you have the mortgage and when payments are due. It is your responsibility to make the payments. Payment due notices, late notices are generally a courtesy on the part of the lender.