I think I have identified the issue. I disconnected the diesel fuel feed / return lines from the tandem pump and supplied power directly to the in-tank pump. Diesel came out of BOTH hoses at the same rate, which doesn't seem right. I joined them together by clear tube hoping to see the diesel correctly flow from the feed to the return, instead, I found that both lines were supplied the same pressure - so it leaked a little under my jury-rigged tube but just left an air bubble in the middle of it, ie. no flow.

I disconnected the lines from the tank to the fuel filter housing instead, and fitted my clear tube in between. I ran the in-tank pump again and lo and behold, the diesel flowed correctly from feed to return.

I took the top off the fuel filter housing (recently replaced fuel filter and seals with the correct kit) and couldn't visually see any issues at all, but something is definitely wrong inside the upright centre section of the housing.

I've fitted a small mower-style inline filter in place of the filter housing for now, it's probably not adequate at all, but it should be sufficient to see if I've identified the issue. Will report back how it goes

I can't find any reference online to disassembling that centre section of the filter housing to examine it for issues so I might just be looking at finding a replacement...