In reply to BarrySW19:
Being able to see the bags with warm jackets in was the real kicker!
Dick's Climbing in Bristol seem to have the cheapest ab rope I've seen at £1.30 a meter. I'd suggest much the same as other people have with regards not using your long ab rope for setting up top ropes as they don't tend to be particularly light. 15-20m would be my suggestion as a rigging rope, mine is 15m long and occasionally comes up a little bit short, but if you use a couple of slings you can usually get away with it.
Be careful with the diameter as well, 11mm is a bit on the thick side for some belay plates, but will be harder wearing.
I tend to use a 65m ab rope, but that's more because I can borrow it from my uni club than anything else!