In reply to Robert Durran: I am not sure what you do with you ropes indoors but I've always ended up with ropes that are fairly evenly worn across their entire length rather than at the ends. As such I would never gain anything much from starting with a longer rope.
Also, at £40 for a 30metre rope, you would only be saving perhaps £5 max by buying 60metres, since the cheapest price I can see online is about £70.
However, I think the downsides to longer ropes are considerable. I would be continually pissed off with spending extra time pulling it through to top-rope routes at the wall and when swapping ends after falls. They take considerably longer to uncoil and/or backstack. Finally, you massively loose out on not having the flexibility Hewitt mentions of having a short single rope for going cragging.
I think a short single is one of the most useful and best value bits of equipment you can get.