Can I just fit a cassette with more gears with the same derailleur? I've got a Boardman Road Sport which I use for commuting and doing reasonable length rides during the week etc. I have Shimano Claris as my rear derailleur and so is some of my groupset. I would like to upgrade my cassette to an either 9 or 10 speed to give me a greater variety of gears. To do this do I need to change my rear derailleur from 8 speed to Shimano 105 or something similar? Or can I just change my cassette? Do I also need to change my shifters as well?
If you want to change your cassette to something with more gears, then you will need to replace the rear shifter and possibly the rear derailleur.  A cheaper option would be to get a wider range cassette which still has 8 gears.  You'll have bigger jumps between the gears, but you'll have a bigger range of gears.  With a wider range cassette, you may need a new rear derailleur with a longer cage.