Again, thanks for such a detailed answer, and thanks for being so patient with me! I do realise I'm going about this the wrong way round in many ways - trying to find an industrial scenario to fit the sort of model railway I want to build and run, rather than building the model to depict the genuine industry. I also admit to free application of artistic licence and a large dose of what-iffery. I am willing to suppose that the industry developed in my own idiosyncratic style in my private fictitious corner of Cornwall! All the same, I'd like my model to have some basis in reality.
The passage I was referring to in Maurice Dart's 'Industrial & Narrow Gauge etc' book is on page 25, under the heading 'Hendra Light Railway', he states "This 2ft gauge line at Nanpean transferred clay from the coal fired drying kiln three quarters of a mile to a storage building alongside the West of England China Clay Co's siding near Drinnick Mill. A loaded train is leaving the kiln c1922 with clay bound for the store.' I certainly don't want to dispute your far superior knowledge (compared to mine) but there does seem on the face of it to be some discrepancy. The trains in the photos are made up of tipper wagons, but the images are not clear enough to make out what those wagons contain. One would imagine bagged clay, but that seems unlikely to be carried in tippers, unless they were just using whatever they happened to have available.
I am aware that there would be a return flow of materials as you describe, and I could always assume there might be another type of business using the system. I have MJT Lewis's book 'The Pentewan Tramway' in which he also refers to supplies to and from lime kilns at Pentewan and St. Austell. I know this was also an industry served by the Looe & Liskeard line. Indeed, I had thought this might offer an alternative primary industry for my layout, but I am finding the clay industry more fascinating the more I look into it. I might end up compromising and modelling an exchange yard (wharf, if that's the correct Cornish term?) between the GWR and a narrow-gauge line serving several local industries. From an operational perspective this might prove both more interesting and more practical, given the space I have available.
The other book I ordered, which I couldn't remember the title of yesterday, is 'From Lostwithiel to the China Clay Rails' by Bernard Mills. That arrived this morning and looks to have a lot of useful photographic reference material. I'll have a good read of it as soon as I've finished the one I'm reading at the moment about the 251st Tunnelling Company in World War One! I might be a relative newcomer to the county, but I'm immersing myself in local history!
Once again, thanks for all your help.