Building a fence on top of existing wall - some questions

Hi - I orig posted this in woodcraft, might get some more action here,
I'm building a fence above existing perimeter walls in suburbia.
Exisiting wall height is approx 1.6m high and I plan putting ~600mm high fence on top.

The wall is cavity block with capping on top with a lip/overhang of about 30mm.
Constructed in the last 6 or 7 years. seems a solid construction no movement/shaking etc.

The fence itself will be lengths of larch or cedar cladding (mix of 144x18 and 94x18) configured horizontally with ~10mm gap between them.
is the gap sufficient? I want to keep it private as possible, but also want to allow wind to blow through so i don't make a sail of it.


I plan on screwing 1.2m lengths of 2x4 vertically to the wall face as battens - 600 above, 600 below.
Is this adequate contact with the existing wall?
I'd like to use 2x4 planed larch/cedar to match the fence timber, but should i be going with tanalised/treated timber?

I will use an angle grinder to notch out the overhang on the capping so the batten is flush/plum and squirt some outdoor silicon or whatever in and around where the capping now wraps the batten - so it can still do its original job.
Is that the norm?

The longest run of wall is about 10m. I plan on putting the 2x4 battens at 1.8m centres.
Will that be adequate to hold it?

Anything else I should consider?

Thanks.

PS: Just to clear up on planning - I'm aware of the planning laws, Its to the rear of the house, I don't have planning, the neighbours on the other side are looking for this, we are all in agreement. If the need arises, or I get in trouble, I will take it down or reduce the height, or apply for retention.