Clive!!
The rear speakers *ideally* should be placed around 12 inches above head height from sitting position, so 6ft could actually be a little high. I say *ideally* because very few people actually have the *ideal* positions available, due to the geometry of the average living room. The best way I found was to throw the book away, and have them where they sound best to you. As I've said before, the downside of a Home Cinema is the fact that there is only one 'perfect' seating position, so you just have to make the best of what is practicable (and agreeable with the wife!)
The sub-woofer can be placed virtually anywhere, because the low frequency sound is not directional, but again *ideally*, it should be placed somewhere in between the two front speakers. It's fine under a desk, as long as there is room for it to 'breathe', and the bass port is not blocked. Again, have it where it sounds best to you.
Just a thought, I don't doubt that they would be, but make sure any speakers, including the sub-woofer, are magnetically shielded, before placing them near the TV.