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its happening again... tree is laden with green plums, but instead of ripening, some are starting to go brown and manky... (brown rot?) is this the heat followed by wet weather?

It did this a few years ago, and the tree has never really recovered and cropped well since.. I was hoping for a nice crop this year.

Is the tree irreversibly diseased or will discarding the manky fruit be enough for the undamaged plums to survive??

Brown rot seems to come with cool humid or wet conditions and just as the fruit is starting to ripen it sets in. The tree isn't diseased (as far as I know), it's just one of those irritating things that strike, like blight.

If you fruit is nearly ripe you could try picking a few of the good ones to try to ripen indoors but in my experience if it is brown rot it won't stop them rotting indoors either!

Ripening in the indoors might work as it comparatively dry and warm

I tend to snip the rotten ones off as soon as I spot them, and pick as soon as they look ripe (or just before!!)