How should the tribunal interpret the "necessity" criterion?

Your point about “such treatment” is how I’ve always tried to explain it:

I was just trying to find a way for my wayward tribunal to have been correct, and ended up quite liking it!

And, yes, it’s funny that we’re belatedly discussing this just before it all gets changed anyway. The 1959 Act wording was clearer (check out s25 and s123). The tribunal just had to consider mental disorder (no nature/degree mentioned) and necessity (health/safety/others), and the necessity test was explicitly about whether “the patient should continue to be liable to be detained”. (Nature and degree were mentioned in s25 and I guess added to the discharge criteria partly because of Winterwerp’s “kind or degree”.)