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Paul, having finished work at 9, I managed to do my shopping before the 10 pm shop-closure.
Having had a look yesterday, while on the Heights coming back from the meeting, at the walls which are pictured above, my assessment is that anything growing between the bricks must be cut off before it can cause further damage.
For that one needs a long ladder, or a van-operated crane-basket (this is a much more expensive option though). Cutting the grass at the top of the walls in that area is also a risky job, as the walls are high, and the ground slopes steeply towards the top of the walls.
I'm not sure if volunteers are simply allowed up there just like that, and to do this sort of work, one needs to dedicate many days of work, it can't just be done with a few hours here and there.
There is also a railing going up the steps, it's rusty, and needs painting.
It can't be done with just a few hours here and there.
But that brick does need removing or re-setting with cement, and the one next to it.
There is an immediate danger of it coming down and hitting someone.
It's only those two bricks, Paul, the rest of the walls there are stable.