Generally max heights are 1 metre at front of property, 2 metres at the rear.
This also applied to hedging (and the dreaded leylandii) until Prescott (deputy PM) got hold of the rules, screwed with them, and tagged the High Hedges Law onto the ASBO legislation.
Now if you feel your neighbour's hedging is too high you have to PAY £hundreds to make a complaint, then hope the local council's tree inspector does his job and slaps an ASBO on the neighbours forcing them to keep the hedge to a maxiumum height he determines.
But here in Cardiff you pay the extortionate fee and wait, and wait, and wait.......
More info and help here:
http://www.gardenlaw.co.uk/ and
http://www.gardenlaw.co.uk/phpBB2/index.php