Yet another Borrowed Neutral Problem .............. sorry

My Tain of t
Im doing a remedial Contract for a Local Council following up on problems highlighted by Test engineers.
My Job Is to fix broken sockets, Pendants, change consumer Units, Add lighting crcuits to RCBOs etc etc I then follow this up with a minor works cert.

All good so far.

Ive read up on the dreaded borrow neutral Posts and realised ive been doing it all wrong, when trying to rectify the problem.
My Train of thought was to drop all the pendants and test all the neutrals until I found a link between the downstairs lighting to the upstairs lighting, split that link thus splitting the lighting into 2 circuits problem solved, there are 2 neutrals 2 lives and 2 earths at the CU, I now realise this is not the case, wasted a few hours to find that out.
Typically the domestic dwelling has 5 lights on the down stairs circuit including the upstairs landing light, and then 4 lights upstairs.
The Houses in the main are all wired in 2 plate using double insulated singles, that council have specified that all future rewires to any properties needing re-wires are to be wired 2 plate .... !! (I understand why although I don't agree)

I understand tp a point how the borrowed neutral works, I don't fully understand it but slowly getting my head round it.

Rewiring the Lighting is not an option, doubling up on the RCBO is also not an option therefore my question is how do I fix this problem quickly and easily causing as little disruption to the tenants as possible.
Im in some pretty rough housing areas so the less time I spend in these properties and get moving to my next job is beneficial to me.

I get that you guys are pretty bored now of answering these questions but any help or advice would be great, and thank you in advance