Noted on language ChloeWright. If he/she could do the job as required today, age would not be an issue.
There is no JD, as such, just a very old contract, that has never been revised. There's a move to get a new contract, conforming to NALC guidelines in place, but the Clerk isn't engaged, which means that the Council is prevented from functioning effectively. From what you've said, this seems to be the right approach. However, again from what you say, there's no argument regarding underperformance because there is no JD. Is that correct? Worth noting that training and support have been offered but rejected by the Clerk as unnecessary, I think - in part - because currently, training would be online, and that would be an issue.
There's a lot of resistance to change, which wasn't addressed when it needed to be in the past.
Perhaps the best option then is redundancy, because the role as currently performed is redundant. The Council needs somebody who has the skills to do the work efficiently, or the Council can't be offering value for money.