Tips for removing worn chainring bolts?

[raleighnut]

One of the video makers did warn about using cheap, nasty allen keys. I noticed that some of mine don't look very good!

Assuming Torx then, what size do I need? (I'm assuming that I can buy one somewhere local rather than a complete set.)

I'd be looking for something like this at around a fiver

 

[cheys03]

^ good shout. e.g a quick look over Torx sizes vs metric Allen/hex suggests that an M4 might be defeated by T25, T27 or (with a bigger hammer) T30 so having a range will make it much easier to find exactly the right one, dependant too on how rounded the Allen heads in the bolts are.