Mizter T wrote
[...]
I wonder if at Waterloo it's as the gates being coded to reject all
'TOC-only' tickets apart from 'SWT-only' (of which I don't think any
actually exist, do they?)

They exist from time to time.

Like last year's "Midsummer Madness", weekends in August except Bank
Holiday w/e - £10 day return "to anywhere on our network", SWT trains
only.

--
Mike D