Where it is at the moment is a bit inconvenient. There is some dead space behind the telly I'd like to use but I'd have to have the sub on a table or something about 2 feet off the ground. Would it still sound ok? Behind the sofa is a no-no, although I might be able to place it nearby to the side.

It's a rectangular room with the sofa at one end and TV at the other (longways) and a laminate floor on concrete.

That's exactly where my sub is behind the TV on a shelf screwed to the wall. It sounds just fine though you might need to tone it down a tad because of the bass reinforcement from the corner positioning.

Putting the sub on a table may cause some unwanted resonances. A concrete plinth would work or is there some fancy dan sound-deading material that could be used to make the table?

I have a couple of spiked speaker stands so am thinking of putting the sub (which is also spiked) on them behind the telly - shouldn't have any odd resonances.

Not next to the neighbours wall, by any chance ??

Er...yes, but the 36" telly we used to have was there and it had a sub in it, they never complained...

I understood that subwoofers should be positioned on a floor (hence the spikes on them, and a speaker facing downwards) to let the sound vibrate to your feet? Is this description all wrong?

Yes, the function of a sub isn't to shake the floor

Not all subwoofers are downward firing.

And not all hifi systems require a sub. But you do need Front Right/Left floorstanding speakers.

The sub for satellite style little boxes is needed to fill out the frequency range, and often reach well up into the mid-frequency band, making music less precise in the stereo domain.

No, the more sturdy the ground around it the better for its sound output. Being on a shelf or away from a wall lowers subwoofer sound output efficiency and probably adds bad resonance problems as said. Being in a corner reinforces the output the most, but not to say that is the best location. The normal trick is to put the subwoofer on the ground where you'd normally sit, then walk around the room to where it sounds best, then reverse the positions and thats it. Now this works with quality subwoofers. Ones that are driving just low range frequencies, cheap or small units will spend their time outputting lower mid range sound which is directional, and that limits your options to somewhere near the tv. http://www.polkaudio.com/education/article.php?id=20