Thanks for the replies, but how is it possible to ride the bike and play with the barrel adjuster at the same time? Another problem is that the derailleur barrel is broken and won't turn at all... but according to parktool.com, the shifter barrel offers the same degree of adjustment as the derailleur barrel, so I guess I might be able to do it anyway?...
I went on a purchased a new rear mech off ebay, Shimano Tiagra, just in case the one currently on the bike has had it...
I'm thinking that if I manage to sort this out then I will learn quite a lot about how the derailleur works and I'll be able to troubleshoot any (most) other problems that arise in the future. This is probably the learning curve for me.
Again thank you all for the time you spend to answer my stupid questions!