Based on your answers, and since you grow indoors like me, I can share what my experiences are from growing Cattleyas over the last two decades (in sweat and tears).
I find that cattleyas will grow well in any medium up to 2 years, when the mix starts getting too acidic. The orchiata has a tendency to do this, particularly if flushing is not in your regiment. I agree with SB to include perlite and activated charcoal into the mix (as I do in ratios of 3-2-1). This will keep the pH more adaptable to the roots. Do checked the pH of the outflow as well as the pH of the water/fertilizer trends (ideal range is 5.8-6.2 +/- 1 pH). Sadly, I didn't do this for years and my pH was 4-5 with the fertilizers and I was killing roots (even though I use RO water). My plants went downhill and suffered all ailments (from rot to root loss). Once I remediated the media and pH, all was well and roots grew like crazy.
I cannot stress the (importance) use of a mini fan or room fan to circulate stagnant air around the plants indoors, especially around the heaters and cold windows. The pockets of air can induce mold and microbial growths, holding on to condensation and leaf wetness in areas you may not see. I have seen my plants succumb to new growth black rots (like yours) due to this. It disappeared when I had fans on plants 24-7 with a gentle breeze. Likewise on hot days with the sun blaring down on my west facing window, the plants heat up to almost 90F and can overheat without the fan. These micro niches are more dangerous than most people think and can weaken the plant over time. That's why by the time we see the rot, it has already been there for days and maybe weeks, spreading through plant areas you can't even see. That's why cutting off visible plant tissue is not a cure. And remember that mold is everywhere!!!
Another indoor cultural mistake I made (and seen many people do) was using communal collection trays for several plants. These trays are harbor not only wastes but bacteria, viruses and molds, most are harmless to healthy plants. But if a root of another plant has been damaged by pH or other physical method (such as during bloom staking), it can enter and infect. This infection is usually staved off by healthy plants, but a continuing stressed plant from, let's say, the above items, they can succumb to it. I have seen an entire tray of 8 adult cattleyas killed by rot in a matter of 2-3 months, while the next tray had no effects. What I do now is to have each cattleya plant on the shelf with their own shallow plastic trays. This helps keep other plants' waste water from them as well as to allow longer water absorption (dries in 2 days).
Part of my feeding routine is to use supplements that can improve the immune function of the plants. These include rotations of kelp, aspirin, beneficial bacteria (Sub. futilis), mycorrhizae, Hygrozyme and B1. These do not replace the feeding of complete fertilizers and minerals, but rather act as vitamins.
If I do find rot or disease in my collection, here is my system:
1. immediately isolate plant
2. cut off affected tissue
3. treat with peroxide, physan and/or phyton27 copper (also spray areas and plants in close proximity)
4. dry plant in pot for 2-3 days
5. then gently remove partial media to see roots. If alive, put fresh media. If dead roots, remove and cut off, repot in NZ spagnum moss, cover in plastic, set in warm area)
6. retreat plant every week
The success rate of this is less that 50% because as mentioned before, the disease part of what you see is just the tip of the iceberg.
Anyways, my 2 cents (or pennies as you say in UK) lol...