Rat Pedal

I don't think they are much good for tightening up anything. The Rat is more of a very fat and raunchy distortion sounds great on bass for metal and for guitar if you like the dirtier kind of sounds.
 
Perfect for bass, sounds like a buzzsaw on guitars.

I'm trying to get my hands on one that doesn't cost as much 2nd hand is it does new (they seem to hold value too well in the UK).

Try to get an old one, alot better than the ones you buy new today. I had both and the difference was quite clear. The sound on guitar is less buzzsaw than the HM-2 by the way, but these two complement each other greatly to provide one big wall of grinding nastiness hehe.