Guest Deni C
Richard Bridge (above) is correct.
Any PERMANANT extension to ANY electrical system must be certificated by a qualified electrician who ALSO holds the necessary certification qualifications (not all of them do - it means courses, tests, and another bit of paper). This includes do-it-yourself installation.
If it is only a plug-in extension you will be limited by a maximum 13-Amp fuse in the plug.
As has been advised, you would be advised to talk to the Association of Festival Organisers and your Insurers.
If you do use a plug-in cable you should be able to obtain sufficient length in one reel from an electrical wholesaler. Far cheaper (and safer) than several short lengths plugged together. The wholesaler I use is TLC, who do a delivery service if you cannot get to them. Look in your Yellow Pages under "Electrical Wholesalers".
Colyn.