I've read the article Parksy refers to and also the Ofgem guidance for re-sellers of electricity (effective from 2003 - revised 2005 - believe it or not, doesn't time fly as you get older!!!!)
My concern is that there are still siteowners who are making a separate charge for electricity, in the case of this one I was referring to £3.50 per day (charged from 11 am irrespective of staying the night) for seasonal and £4.00 per day for tourers, advertised on publicity material, and I think the siteowner is opening him/herself up to the possibility of legal problems. Particularly as the Ofgem guidance states that the siteowner must not charge more than the cost of net electric plus 5% VAT irrespective of the business paying 20% and climate levy. The Ofgem guidance also gives examples of how the costs should be worked out and if estimates are more/less than £5 over the year or £2,50 for six months the difference should be levied or refunded.