My 242 has more play in the steering than i care for, especially when trolling along.

Now before i head off with my shopping basket, is the steering bush replaceable on the steering head? Or is this a sign that the head unit needs replacing?

Further more, do the teleflex cables develop significant wear on the drive part?

You always get some play from the helm on a single cable setup - but the biggest problem is from play between the gimbal ring and tiller arm. Get someone to push/pull the drive side to side, and if the drive moves and the tiller arm doesn't, that's the problem. It's a common occurence on bravos. There are some quick fixes, you might be lucky and it's just the clamp bolt either on the gimbal ring or the tiller that's loose, but if they are worn significantly, the proper fix is to pull the transom assembly and repair/replace the worn bit.
There are some 'fixes' that involved drilling the side of the transom plate to get access - but I'm not keen on that kind of repair.

On my boat it was the tiller arms that had worn, and is one of the motivations for fitting hydaulic steering now.

Excellent diagnosis Mat. I went through the same thing last year when I did my engines. You can try tightening the u-bolts (the two nylon locknuts at the top of your outdrive) and it will usually help - for a while. Just don't overtighten them. As Mat said, the eventual fix is to have the gimbals/transom assys looked at. The drilling of the transom assy to get at the tiller assy is a mercruiser approved repair but you wind up with 2 large pipe plugs on either side of the upper transom assy when you're done. Of course, the other option is to yank the motor so you can remove the transom assy. Pick your poison.