So I have connected the humax to another TV. Brand new Sony. Seems much better, but I still occasionally get the handshaking issue.
My conclusion - this box has problems with pretty much every Sony TV that has been been made in the last 10 years.
Now, Humax could point the finger at Sony, but it seems odd that they would engineer their product in such a way that it is incompatible with at least one of the market leading brands out there (and probably with others too).
Not impressed at all