I have found a briefing paper and further information that leads me to conclude the following:

 a) the part of Cumbria already supplied with drinking water by Northumbrian Water, so under threat of fluoridation, is 'Alston Moor' - comprising Alston, Garrigill, Nenthead and 8 small hamlets with a total estimated population of 2,345 residents

b) in Darlington a total of 2,589 residents , but only 31 in Stockton-on-Tees, plus 1 or 2 properties in Gateshead and Hartlepool are also under reported threat

c) Sunderland (290,016 residents) and South Tyneside (157, 938 residents) Councils are now bound to respond to a letter from Durham County Council requesting their consent to join a Joint Public Consultation, scheduled for 9 May to 9 August 2020. So, if the respective Borough Councils do both accede to this formal request, an estimated total of 447,954 plus an extra 201,739 County Durham residents are due to be consulted, starting in May. [Total = 649,693]

We can but hope that enough households can be persuaded via local media responses to vote to reject the proposals, because they know enough to decide for themselves that the last thing anyone should vote for is to be poisoned daily and lifelong via the water supply in return for essentially trivial dental health benefits at considerable taxpayer and local expense. The current lack of any locally organised antifluoridation groups represents an unprecedented campaigning situation, I believe. [I don't think NPWA will be able to do enough in present circumstances, including communication costs, to inform/motivate local residents sufficiently]. 

Ian P.