In reply to philpaynter:
>"towers that are good for abseiling"
Nelson's column in Trafalgar Square, London is quite good.
You'll need 2x50m ropes. A 120cm sling goes nicely over Nelson's hat and wont slip over his shoulders. This can be backed up with a thread through his bad arm and should be if you are intent on using the in-situ sling around his neck which was showing signs of where the last time I was there. The platform on top of the column can not accommodate more than 14 people so it isn't suitable for very large groups.
In the past there have been severe access restrictions however since the BMC slipped Boris Johnston a bung things have relaxed somewhat.