By the sound of it, you may have missed the boat. When a vacancy arises, a public notice must be posted, stating the set period during which ten electors can request a poll. If ten electors (they do not need to act as a group, they can simply be ten individuals) make the request, then a poll is held. The town council will have to pay the district council for the cost of running a poll.
If the set period for requesting a poll has passed, then the council is entitled to fill the vacancy by cooption. You should be able to find out the current status by asking the democratic services department at the district council (or other principal authority).
Cooption is clearly less democratic than a poll, but it is a lot cheaper.