I took a new approach on Tuesday and used the email link on their website
https://www.scottishpower.co.uk/contact-us Using the General Query link.
I had a response the same day saying they would be in touch within 5 days.
They called me on Wednesday and said they would send a key Special Delivery that day.
It arrived this morning, so I am impressed that this worked.
I made sure that the emssage I sent had all A/C numbers and details of why I was eligible, I quoted the various responses I had had from the call centre staff and made it clear I was not blaming them at all for the delay.
This may be a worthwhile alternative for any of you who have been trying to call them, good luck.