Thanks for getting back to me.
I actually did a LOT of Googling and asking around and eventually stumbled upon this blog: http://tuxtweaks.com/2010/10/remove-...h-one-command/
which seems to have achieved what I need! I now have 34% free in /boot.
One thing I noticed was that a number of people were saying that /boot does not need to be a separate partition, yet this is how Ubuntu sets itself up by default...