Chain overshoots cog when shifting from low to high gears Hi when shifting from low to high gears my chain is overshooting the cog and jumping into the next one. I tried turning the low limit screw outward to move the derailer in towards the low gears but it makes no difference so I think it must be the cable tension. With wear and tear I am guessing the cable has stretched so needs tightened at the barrel adjuster and not loosened? Thats my conslusion but maybe it is not that simple?! Any advice would be great, thanks. Dont want to mess with it too much before being sure incase it completely messes it up as I have done many times before :)

Decided to go with what I thought. Adjusted the rear barrell adjuster anti clockwise 4 clicks and the chain started overshooting in both directions (mainly at the low end). Undid the 4 clicks and the chain now overshoots in both directions at the low end yet hesistates to change when shifting from high to low (at the high end). Im summary gear adjustments are a dark art as I previously thought!
The following links will show you how to adjust a derailleur: 

http://sheldonbrown.com/derailer-adjustment.html

http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur

Note that cables and derailleurs do need replacing from time to time and they do need occasional adjustment, but try to follow one of the derailleur adjustment systems suggested in the prior links (assuming the derailleur isn't bent or something - if it is, take it to your LBS to be straightened). Limit screws only keep the chain going too far into the spokes or out of the cassette, and do not affect shifts between the largest and smallest cogs (only the extreme points of chain movement), so those should not have been tweaked (probably). 

Note that bending a derailleur is quite easy even with a small spill. Your LBS probably can adjust this in 5 minutes if you can't figure it out.