Thanks everyone for your input, just been looking at Regs book and I think 537.2.1.1 confirms this:Every cct shall be capable of being isolated from each of the live supply conductors. In a TN-S or TNC-S system it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliably connected to earth by a suitably low impedance
So on boards to 16th or previous, you would have 100ma time delay as main switch and that would have provided the fire protection to all ccts, but then in addition a 30 ma rcd for sockets etc.
You are not quite right on this reg cuz that is NOT applying to TT.
however.. some of the bits you would be interested in are...
Reg 531.2.1 page 112:
An RCD MUST disconnect all LINE conductors at near enough the same instant!
531.4.1 page 113:
If a TT installation is protected by a single RCD it shall be placed at the origin.
411.5.2 & 411.5.3 page 50:
Fault protection can be provided by either RCD or overcurrent protection device.
where low Zs cannot be met RCD's preferred option.
Page 16 of OSG is part of the "general layout of equipment" section...
more to do with Meter/Bonding & earth arrangements etc.. IMHO!
Pages 24 / 25 have the example of compliant RCBO installations TN & TT!
The reg 531.4.1 page 113: single RCD TT installations would not generally meet the reg 314.1. (iii) page 39, in your average house insatlltion...
BUT if it was a small garage or outbuilding that had a TT rod installed and a small 2-way CU with light & socket circuit.. a single RCD can be considered suitable to protect the garage/shed installation in which case this MUST disconnect BOTH conductors because it is ALSO the isolator switch for that part of the installation. and needs to disconnect ALL 'live' conductors (inc neutral) 537.2.2.1
But a house with multiple RCBO's doesn't need to diss 'L' & 'N'