After further research in the VSA records I found that the downstream passage has indeed been explored beyond my far point. Peter Ackroyd in 1986 reached the end, about 9m onward and 3 or 4 metres downward, where he reached a final and seemingly impenetrable sump. The way to there is EXTREMELY tight.
Unfortunately, therefore, it seems that we will probably get nothing more out of this cave. I do plan sometime to follow the muddy upstream crawl to its end (it was dug out in 1963). However, I wouldn’t expect it to go any further without excessive effort, and of course we are not as interested in that upstream direction.
M65 remains one of the more interesting Murrindal potholes. I plan to publish a description in Nargun in the future.