I still can't understand the single live from landing switch to loft. It can't be borrowing a neutral since there is only 1 circuit in the loft. How can it work without a neutral?
With the disclaimer that I haven't tried to follow your written descriptions the way it can quite easily work is that a permanent live goes to COM in the hall switch, and the switched one for the landing light goes from the COM in the landing switch up to the loft (as per the top left diagram in the two-way Wiki article), where the upstairs lighting circuit runs, and a neutral is taken from that. Was a very common way of doing things in the past when there was only one lighting circuit in the house, and was no problem at all until sometime later it was decided to split the lighting circuits into separate upstairs/downstairs.
I'm not sure it can be a shared neutral if that can still be live with the circuit off. I have had this particular lighting circuit off alone and changed the switches over without being electrocuted.
Rat's-nestiness can make it a PITA to find out what the hell connects to what, where (one reason your electricians want to quote an hourly rate), but the underlying principle of how lighting circuits work is actually a lot simpler than it at appears when you first look at all those wires.
Another reason you'll struggle to get a fixed-price quote is those cut-off earth conductors. Who knows what will be needed to reinstate continuity. In extremis cables may need to be at least partially dug out from the walls.
So I've got about half way through my loft reboarding and clearing exercise in readiness for the electrician, and an interesting finding so far is that a single earth cable that I could see go to the lighting junction box can be traced back to a bedroom socket! I suspect the lighting circuit is therefore actually earthed but that it's borrowed from the upstairs socket circuit and therefore continuity would go back to the socket MCB. Could that be the case? Obviously I will tell the electrician about this. All boards so far labelled up clearly too.
Bang goes that theory then. Probably still worth telling the electrician though. It appears someone has tried to earth it in the past using a socket, which I'm not sure if you're "allowed" to do or not. The wiring is such a bodge, there are old cut cables all over to confuse things further (I hope they're old and disconnected, I didn't want to find out). I can't understand why the previous owners replastered the whole house, redecorated it all, and left the wiring in a mess underneath.
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