If the trees are overhanging a public footpath, the district council can force her to cut them back. (If she doesn't do so, the council can cut them back and send her the bill). I know this because I'm the world's worst gardener and I get at least one threatening letter every year, from our district council, about a large shrub which overhangs the public footpath :-)
The order to cut the trees back, so that they don't overhang the footpath, won't force your neighbour to cut back those parts of the trees which overhang your property or obstruct your light. However, if the trees are evergreens and cause you significant problems, you can ask the council to make a 'high hedge order' which will force your neighbour to cut them back. You need to be aware though, that the council will charge you a fee considering the order (which they're not obliged to make if they don't think that the matter is serious enough):
http://www.direct.gov.uk/en/HomeAndCommunity/W hereYouLive/NoiseNuisanceAndLitter/DG_10029999
Chris