Thanks for all your advice - Sheldon Brown's site is great! Everything explained in thorough detail. What I know about bikes has been mostly picked up from friends, reading really confusing books and/or just figuring it out for myself. Sheldon has it nailed down nicely
After reading his page on derailleur adjustment I see what I've done wrong with my road bike; also I think my mountain bike RD is screwed (I tend to beat the crap out of my MTB!)