After figuring out that the fine white gravel I got on the bottom of the tank was the most likely cause for continuously high PH and KH values I decided it was time to do a major overhaul.
Is what I had on the bottom. After some serious head scratching I decided to test it. Put it into some vinegar and after 72 hours it was completely dissolved.
So, time for something new. I bought a bucket of Dennerle Deponitmix
Which I know is long lasting and shrimp compatible. I also got two bags of Dekoline quartz . The dekoline quartz is obviously quality stuff. Contains very little dust and no debris whatsoever. Easy to work with. Just to check I put some into vinegar as well and after 72 hours it was completely inert.
So, d-day. Got some buckets ready and a big tub for the plants. Filled one with water from the tank and spent considerable time catching every little shrimp. Even found some in the filter who'd been without water for at least 15 minutes and who happily wriggled away again.
Four hours later I'd removed the old gravel, rinsed out the tank somewhat, poured the new substrate and gravel and replanted the tank. I've decided to give the Hygrophila polysperma one final chance. Planted a dense clump of it. If that doesn't work I'm going to replace it.
For the next week at least I'm not even going to touch it, just see if the water values stabilize. After that I'm going to do a phased restocking.