They could possibly come within the scope of the high hedges legislation. If you've asked your neighbour nicely to reduce the height and had no response then have a word with your local council planning department.
Be prepared to lose any good relations that you have with your neighbour if you go down that route.

2 metres or 8ft is what is considered. If you want them shortened to this height then you need to ask the neighbour and if they say no to go to the local authority to decide (note this costs £350-£600).

You also have the legal right to cut off any branches that overhang onto your property over your boundary. But by law you have to give the bits that you cut off back to your neighbour because they own them.

That suits me fine because my prune face of a neighbour then has to dispose of her own cuttings. LOL

PS. There's a leaflet that might help at https://www.gov.uk/government/uploads/system/uploads/ attachment_data/file/9411/highhedgescomplaining.pdf
(Copy both lines and remove the space between uploads/attachment)