Many thanks for getting back on this, rebooting the V6 did clear the problem.
My V6 was changed yesterday as I've had a few other niggles with the previous box, and touch wood the new box is working absolutely fine.
However one thing that did come to light yesterday was that my original V6 wasn't installed correctly when first put in and according to the engineer it was running old TiVo software. He felt most of my issues were a result of this, he did start to update the box but he estimated it could take as long as 90mins so just swapped boxes.
Perhaps this black screen issues could be a result of poorly installed V6 boxes?