Thanks everyone for your insight and experience! Really cool!.
Theses are my current thoughts regarding expandable TOC's.
1) Most of the books I buy don't have an appropriate TOC so I nearly always have to do it myself in Calibre.
2) If an OS and/or E-reader is able to recognise an expandable TOC, the ones I've done in Calibre always work.
3) Android and Moonreader recognises expandable TOC but I either have to use on a 'tablet' or 'smart phone' which is not easy to read in sunlight etc. - or I have to put android on an e-reader (ink) which seems to be sluggish and drains the battery faster than normal.
3b) The Illumina 6" E654BK is an android E-Ink reader that might be the Holy Grail but their website doesn't mention how many hours of battery life can be expected - whereas 'hours of battery life' is a MAIN advertising component of all other e-readers - meaning that the Illumina will almost certainly have a low battery life....
https://www.icarusreader.com/index.p...product_id=239
4) KoReader on Kobo regognises expandable TOC's but the downside is no 'desktop' - essentially it's a filemanager. (This is currently my best option and I'm 'content' with this)
5) Coolreader does not recognise expandable TOC's
6) A generic Kindle Paperwhite reads expandable TOC's - downside is the KP has no option for more storage (SD card etc.)
So as far as I know currently, apart from using android and an app, the ONLY e-reader that can natively read expandable TOC's is the Kindle Paperwhite but any e-reader that can run KoReader can too.
My preferred choice will be to go and buy a Kindle Paperwhite - which hurts..........
Saying that, the Kindle store still sells most books with terrible (or no) Table of Contents and so hours of editing in Calibre is needed irrespectively in order to sort it out. I've been thinking of 'selling' the book back to the publishers with a proper TOC so that they can 'update' it in the Kindle store so that other users don't have to go through the same process as I have to.....