My lcd tv is fixed to the wall.
I have a out-of -the -box home cinema system below it
The only cables connecting them are a HDMI cable and of course the aerial and mains cable going to the tv.
My question is would it be a problem to tie the cables together for neatness without causing any interference ie. mains,hdmi and aerial, I will probably add Sky hd later,
also do I need to connect a optical audio lead?
Many thanks.

Should be no problem at all, and it's how almost all wall mounted sets are wired.

The optical cable probably doesn't need to go to your TV?, just from the HD box to the 5.1 amp.

But if you're plastering the wires in the wall, ensure you fit every cable you might ever envisage a need for.

I would also suggest leaving plenty of spare wire at the top - consider what happens in the future, when you come to replace the TV - the sockets are almost certainly going to be in different places on the new set, and the existing wires might not reach.

Many thanks.
Will do as you suggest.
I have left it to late to bury the cables in the wall, is it possible to get a suitable sheath or cover to hide the cables?
Thanks again.

You could always box then in neatly, and paint over it - trunking is available, but you probably need a substantial size?.