Over and out...
Today I received a new HDD and happily formatted it, installed it and... nothing
Meanwhile, I reformatted the original HDD (the one that had had a spark of life), then swapped them: nothing, nada, rien de rien.
So I guess it's the main board. Humax say they want £120. That's quite a lot of money but cheaper than the £220 for a new one.
A week of evenings wasted, but Amazon is still my friend 
