Regarding potassium: from the info you have sent the minimum ppm for K should be 35. Is this a happy medium for most turf species?

I would not describe it as a happy medium. I would say that the current MLSN guideline for K of 37 ppm is the number that we want to keep the soil above, for all turf species. One should make fertilizer applications that will ensure the soil K does not drop below 37 ppm. And that is a number that works well for all turf species. Keep in mind that when there is 37 ppm K in the soil, that is about 5 g K/m² in the top 10 cm. That amount, the MLSN guideline level, is like a buffer of K, one that will never be accessed by the roots, because the fertilizer applications will be made to ensure that 37 ppm never needs to be used. Of course, if the soil K is high, then the grass may be able to obtain all the K it can use from the soil, and no K will be required as fertilizer. You can check this, and see how the calculations work, with this Shiny App K calculator.

Would you agree that more K than is necessary may actually cause disease not to mention wasted money, leaching etc.?

Adding 2 or 3 or 4 more times K than the grass can use won’t cause disease, but such rates of K do have some association with increased susceptibility to certain diseases. If the grass doesn’t have enough K, there can also be increased susceptibility to disease. The important thing is to make sure the grass is supplied with just the amount of K it can use, and that can be done by making sure the soil is kept above the MLSN guideline. I would be less concerned about disease, and more concerned about wasted money when applying K that the grass can’t use and the soil won’t hold.

Do you ever see much response in colour, density, leaf turgidity etc from K apps because I have ever rarely seen a response from it in the field?

It would be rare to see a response to K addition in the field. If one keeps the soil above 37 ppm, I don’t think a response will be seen.