I too have those Hornby ruins, I'll add photos in a sec.
BUT I have to say your original posts with the DAS ruins are totally spot on, they are so good I plan to have a go myself and try it as well. Thank you for sharing such a lovely piece of modelling.
Scribing Depron foam can do it, but you always have that surface to disguise.
I use Foamex, which is a self skinning PVC foam used by signage makers. My son gets me copious amounts from 2 mm to 10 mm thick. It is self supporting and does not warp. It carves and engraves with whatever you like for tools and takes paint beautifully. I have so far used it for brick, block and even shiplap walls and have done experiments on it to make a church, built of stone (dressed), stone (rough) and carrstone and it worked beautifully. It also makes very nice paviours and setts (cobbles). I also used it with 40 grit sandpaper rollered into the surface to represent rough stucco in 1/32nd and 1/43rd scales and that, too, works a treat. Engrave a few jagged cracks and you have a one stop shop for all surfaces.
Beg some samples from your local exhibition and /or signage companies.
I sent some to Iain Robinson when he was at the height of his professional modelmaking career and he replaced all use of Das with Foamex.