i'm playing a hofner senator, and one of the tuning pegs buzzes loads. the machine heads are open-back, so i think i can see the problem - it looks like the bit that makes the gear go round (i don't know the words) isn't sitting in the teeth tightly enough, and as a result it's free to move around quite a bit - hence the buzzing and it's also a bit of a pain to tune. the strings seem pretty old, but i don't know how much that could have to do with it. does anyone know a way of fixing this - i really don't have the cash to go finding vintage hofner parts on ebay
Need a pic.

The parts you are talking about are worm and worm gear. The worm is part of the tuning peg shaft itself, and the worm gear is connected to the tuning post (that the string is wrapped around).

Depending on how the tuners are made, you can try tightening the screw that holds the worm gear to the tuning post (not too much or it may be too hard to turn) or, if possible, you could pinch down the retainers that hold on the worm shaft, if it is made that way.
You can sometimes fix loose tuning gears simply by making sure that you always tune up rather than down. - This ensures that the worm is firmly engaged with the sprocket under string tension. Another easy fix would be to bog up the looseness with poster putty - eg between the worm and the baseplate.