To me problem sounds like weak pump. There is nothing in the rack that is temperature dependent.
You were right about the pump!
I decided to just bite the bullet and change the pump, except this time I'm confident it is a genuine OEM pump - see photos attached. It came in ZF packaging but appears to have been made by Bosch - as both the pump and the box also had Bosch labelling and it came with Bosch paperwork. It came with a new reservoir and seal also. Cost AUD$440 + Postage.
I am delighted to say that it works! My ML has had a reprieve, because if it hadn't worked it would have been the end of the road!
The fitting process should have been straighforward, given I had done the job before, but I was thwarted by the flare nut where the high pressure hose goes into the pump - when I went to undo it, the whole lug that it screws into in the pump body turned. So I spent a fortune buying a 27mm spanner to fit the lug that screws into the pump body, and I bought a 17mm flare nut spanner. Trouble is there wasn't enough space to get both spanners on the nuts side by side, so I had to grind down the faces of my brand new (expensive) spanners so they would fit side by side. To my horror the edges of the flare nut started to round off rather than the nut moving. I applied WD40 to it, heated it, then cooled it and I still couldn't get it undone. In the end I managed to undo the flare nut using and adjustable vise grip. I thought I was going to have to remove the pump with the high pressure hose attached, which would have been a nightmare job.
I learned a lesson. Be wary of cheap, non-OEM critical parts (my previous replacement pump cost US$100 and turned out to be useless).