There is no specific height limit to a hedge. However, where an evergreen hedge is over 2 metres, you have the right to apply to your local council for a 'high hedge order', which would compel your neighbour to cut the hedge back to a height defined by the order.
You should be aware that you have to pay the council to consider your application and they are not obliged to grant it. (You would need to convince them that the hedge was causing a sufficiently large problem for them to act. Also, no order would be granted unless you could convince the council that you had already taken all reasonable steps to solve the problem).
The relevant legislation is here:
http://www.opsi.gov.uk/acts/acts2003/30038--i. htm
The government's advice page is here:
http://www.directgov.gov.uk/en/HomeAndCommunit y/WhereYouLive/NoiseNuisanceAndLitter/DG_10029 999
Chris