Zones only came in 1987 with 15th revision.
Prior to that date diagonal was common - less material cost.
- Some electricians ran diagonal if they oversized cables re grouping to reduce material cost
- Other electricians did not run diagonal but grouping was handled by "60A fuse, grouping negated by diversity in domestic"
1951s - strict shortest route diagonal (shortages post WWII) 1970 - diagonal but not strict shortest route 1983 1984 1985 1987 - all slight diagonal, different electrician each time 1994 1995 2000 2002 - diagonal roofline prevented horizontal zone so assumed ceiling line became a diagonal zone and used doorframes as zones (under architrave)NorthWest, Cumbria, Midlands, London.
Additionally PVC in the 1950-1960 period can be affected by green exudate, this is only an immediate issue where it enters wiring accessories and can cause tracking, but should be replaced.
The OP needs to check what the cables are, "at least 45 years old" may mean PVC/PVC or PVC/Polyethylene or some TRS for certain high current fixed circuits (washing machine). Rubber has the best insulation, tinned for corrosion protection, but a shorter life than PVC which can range from 200yrs from temperature data.