I've never worked on a diesel before but this did look like just a worn out battery problem - which does not now seem to be the case.
afaik the compression ration on a diesel is much higher than petrol - but as I say the cranking speed is just a crawl.
Additional clue: I just took it for a 10 min drive then tried a few engine starts. Just *one* of those starts had a slow cranking speed - all the others were "normal". So I wonder - could it be worn-out starter motor? Also thought maybe just a bad connection in the power leads - somewhere? Battery posts/connections are very clean, dry - and look fine. Thanks for any help.