Thanks, I aim to please :)
So far there are a few options for the cause of the problem.
The HDMI port itself is original They are distinctive in their construction) and a known failure point, so that is up next for replacement. Then there are the diodes around the HDMI port. Having tested them against my other board, I'm not convinced that they are right as the values seem to vary considerably, so I am going to swap them with those on my donor board where values differ.
There is a fuse in the circuit too, but that is ok, and in addition I have checked all the caps for shorts and there are none.
I think the HDMI encoder is probably fine, as are the EMI filters, and I have tested the path back through from the HDMI port to the encoder, via the filters and I believe they are all ok, there are no shorts and continuity is good.
Certainly the readings through the differential pairs are identical, so no cause for concern.
So, if switching the diodes and the HDMI port doesn't fix it, the only option left is likely the Southbridge. I can try a swap with the donor board, though I may well just be better getting a known good pre-balled replacement.
That makes me nervous since I've never installed a BGA IC before, but there's a first time for everything, right?

