Hi Tim
Not sure if you've seen the book, "Rails in Wales: The Cambrian Lines", by Peter Johnson, published by Ian Allen in 1984. This may be of interest.
There's a good balance of black and white photos of the lines including many 24s, 25s and the usual DMUs as well as steam. There are some gems including a 128 DPU with an inspection saloon for Peter Parker's visit and the HST visit to Aberystwyth. The local freight and departmental workings are shown as well as the York mail, 24 hauled.
Cheap copies are currently available on ebay.
Best wishes
David
Tauranga, NZ