Sorry for the delay in replying jareth - I am a geochemist so know nothing about electricity! 
Thanks for the link to the WHO report Snorbs.
Have to say I dont read it as saying it has found no evidence of adverse health effects - it mentions several - but what it says repeatedly is that there is currently very little evidence of adverse health effects because there has been very few studies carried out to gather the required evidence. This is a bit different to having gathered lots of evidence that shows there are no ill effects. There is a very long list at the end of Chapter 1 of the research it recommends should be done to address that.
Personally, I am more worried about the neurological effects on children's brains of chronic exposure than cancer and it says on page 16:
"Studies of adult volunteers and animals suggest that acute cognitive effects may occur with short-term exposures to intense electric or magnetic fields. The characterization of such effects is very important for the development of exposure guidance, but there is a lack of specific data concerning field-dependent effects in children...Behavioural studies on immature animals provide a useful indicator of the possible cognitive effects on children. The possible effects of pre- and postnatal exposure to ELF magnetic fields on the development of the nervous system and cognitive function should be studied."
And:
"There is some evidence of an increased risk of miscarriage associated with ELF magnetic field exposure. Taking into account the potentially high public health impact of such an association, further epidemiological
research is recommended."
Exposure limits are almost always revised downwards as new evidence is found - the WHO safe limit for blood lead in children (something I DO know something about) was 10 dl\l for years because that was the limit for the appearance of observable clinical symptoms - then research showed that there were long-term sub-clinical effects (ie adverse effects on brain development in children) at levels below that, and it has now been halved.
Sorry - long post - but I remain concerned because I just don't think we know yet if there are adverse effects or not (or even know what we are looking for!).