Well, that is a very good question, and one that concerns many responsible councillors. I have been involved in a situation where the council wanted to pursue a course that was opposed by many citizens. Two vacancies occurred through resignations, and in that case I was one of the (more than) 10 people who demanded a poll. Expenditure of a quarter of a million pounds was at stake, and so it seemed justified to incur the four thousand or so pounds for a poll in order to have two new councillors with a genuine democratic mandate.
But councillors do periodically leave for one reason or another, and if there are no major issues at stake it seems extravagant to call for a poll, just as you say.
There are no right or wrong answers about how to make cooption as democratic as possible. And no sure fire way to compel a council to handle cooption well.
The kind of factors that I favour include advising candidates for cooption that they must make written submissions that will be available to the public. That way, the council should be making its decision on the basis of information that anyone can see. Anything else that encourages openness is to be encouraged.
Informal polls are tricky - in a small parish they might be viable, in larger parishes and towns it would be hard to have confidence in their fairness.
There is an argument for the selection of candidates for cooption being a ballot rather than a show of hands because councillors will have to work with whoever is chosen, even if they voted against their cooption.
The other point worth making, especially now, is to aim at having a good field of candidates at the elections. The four yearly elections are coming up next May, and that's the time when electors will get their say if there are enough candidates.
