I don't deal with this area of law.But,as a thought:

If you have a legal right to access your water storage tank and the planned extension would stop you doing so,your legal rights can't just be ignored.

In relation to the planning application,you should bring to the meeting a copy of your place's Land Registry office copy print out which hopefully record your right.If you haven't got a copy,see Land Registry website.

You should definitely point out anything like this at the meeting.Try and think through what to say and be as succinct as possible and just do basic bullet point notes.

Have a look at this:
[Link]

Hope this helps.

Edited by legaleagle (29 Aug 2017 8.34pm)

best of luck.
Take notes at the meeting, so you can use for any potential appeal purposes. Get as much information as you possibly can

I disengage, I turn the page.

Planning applications are often decided by the 'technical' contents of brown envelopes. You really would be most enlightened.
The Borough responsible for Grenfell took many monies off developers in order that "affordable housing quotas" could be ignored, and the monies directed into the boroughs social housing scheme,....where it lay dormant in the borough's bank account and still remains unused.

I disengage, I turn the page.