Having made a good effort to get it off, I will be taking it back to the dealer where I bought it nearly new. They are the only ones who could have stripped the thread since it was their demonstrator bike and they performed the first two services, so presumably must have taken the seat pad off. I have never had the seat pad off or tried to remove it till the battery went flat in December.
They’ll probably be able to sort it out for you in 5 minutes ... good luck with it and let us all know ...