Thank you all for replying and especially for making me reread the Wiki carefully. The phrase If Tn is used to select tool n but that tool is already active, the command does nothing. Otherwise, the sequence followed is... to me implies that it always calls tpost0.g but now I know!
To answer specific questions no there isn't a T0 in config.g . As originally delivered by RepRapPro the T0 is put in by the custom Gcode in SlicR from the start GCODE section so I will add an M116 after that. Its interesting that the reprappro supplied SlicR start Gcode for the dual nozzle option doesn't include a T0 and T1. I can only presume that in version 1.09 there was an automatic wait which has gone in the intervening versions. I have looked at the versions in https://github.com/Duet3D/RepRapFirmware/blob/dev/WHATS_NEW.md but that only goes back to 1.12 and I can't see any mention of the change.
There are 2 reasons why I'm building a second Ormerod, first after 20 years as Clinical Director of a large NHS Pathology service I don't tend to make major changes to software without a fallback machine. My plan is to upgrade my original machine once I have a working spare. I also plan to play with dual nozzle so an understanding of tool changes etc. becomes relevant. The reason I'm on 2.02 RC3 was that was the current firmware when my Maestro was delivered. I got sidetracked into AI resulting in me teaching myself Python, Tensorflow and Linux.
My current plan is to is to put a M116 in the custom Gcode setting of Slicr after the T0 and when I move on to a second nozzle to also put in a T1 and M116. Looking at the gcode source of a 2 colour slicing that presumably will trigger the relevant tpostN.g macro each time as that will be a tool change.
Its been interesting and thanks for your input.
Dennis