I feel for you if they are leylandii? as they can grow 3-5ft per year and take water, nutrient and light from a garden .
If you check with your local council it is likely that any Leylandii trees/hedge has to be a max of 2m in your back garden and they will advise you how to proceed.
But a word of caution you need to tread carefully as boundaries can be a touchy subject between neighbours and may lead to misery for you both.
Why not ask her around to see the problem from your side to begin with and try to suggest a way forward it may be that as you benefit too you could offer to pay part of the costs.

"it is likely that any Leylandii trees/hedge has to be a max of 2m in your back garden"

That is not true at all. English councils will investigate (if you pay the £350-500 fee) any evergreen hedge that is over 2 metres high - 8 confiers may not be a hedge and the guidelines are only for evergreen hedges planted after the law was enacted. It will only take into account what is over 2 metres - the first 2 metres are irrelevant. It may or may not request the hedge is reduced over time or may go in favour of the neighbour. There is no law to say hedges or trees have to be 2 metres high!

I do think that your neighbour is being difficult. Having a hedge 2x the house looks stupid and is unnecessary. The more it grows the more money she will have to pay in future. She should have pruned them over time and this situation would not happen.

This is what The RHS say about trees/hedges:

"Tall hedges can be a nuisance, especially where neighbours can’t agree on a suitable height amicably. However, legislation now gives people whose gardens are overshadowed the opportunity to resolve the problem with the help of the local council.
What are high hedges?

The term ‘high hedges’ was subjective until it was defined by the Anti-Social Behaviour Act 2003: Part 8 in 2005. This is a summary of what constitutes a high hedge under the law:

The hedge is more than 2m (approx 6½ft) tall (there is extra guidance for hedge heights on slopes)
A hedge is defined as a line of two or more trees or shrubs
The hedge is formed wholly or predominantly of evergreens (these don’t lose their leaves in winter) or semi-evergreen ones (that stay green most of the year)
Bamboo and ivy are not included
Where a hedge is predominantly evergreen, the deciduous trees and shrubs within the hedge may be included in the work specified. However, a council can exclude specific trees or require different work"
Making a complaint
The high hedges legislation has been designed so that the general public is able to use it without the need to involve lawyers. This would be a simple sequence of events:

Where you feel that a hedge is too tall and affects the ‘reasonable’ enjoyment of your house or garden, the first step is to negotiate with your neighbours. Keep a copy of any letters to demonstrate you have tried.
If negotiation is unsuccessful, contact your local council to enquiry about using the high hedges legislation. There is a fee for making a complaint (typically £400) to deter frivolous applications
The local council will consider both sides’ cases and make a decision
The council will reject the complaint or issue a notice for the work – including the period in which to cut the hedge back and by how much
There is a chance to appeal
It is advisable for the hedge to be cut below the requested height. This will allow the hedge to grow in between trimmings, but still remain below the stipulated height
For more information on the complaints procedure, see the Communities and Local Goverment website: guidance on high hedges legislation."
I hope this helps you.

The government website gives a far more useful guide:

http://www.communities.gov.uk/documents/planningandbuilding/pdf/hedgeheight.pdf

Cornwall Council says:

The role of the council will not be to mediate or negotiate but to adjudicate on whether, in the words of the Act, the hedge is adversely affecting your reasonable enjoyment of your property.

The Council will take account of all relevant factors and will strike a balance between the competing interests of the complainant and hedge owner, as well as any interests of the wider community.

If the Council consider the circumstances justify it, we will issue a formal notice to the hedge owner which will set out what they must do to the hedge to remedy the problem, and by when. Failure to carry out the works required by us will be an offence, which on prosecution, could lead to a fine of up to £1,000.

The Facts:

The legislation does not require all hedges to be cut down to a height of 2 metres
You do not have to get permission to grow a hedge above 2 metres
When a hedge grows over 2 metres the local authority does not automatically take action, unless a justifiable complaint is made
If you complain to your local authority, it does not follow automatically that they will order your neighbour to reduce the height of their hedge
They have to weigh up all the issues and consider each case on its merits
The legislation does not cover single or deciduous trees
The local authority cannot require the hedge to be removed
The legislation does not guarantee access to uninterrupted light
There is no provision to serve an Anti-social Behaviour Order (ASBO) in respect of high hedge complaints.

Before you complain
Satisfy yourself that you have tried and exhausted all other avenues for resolving your hedge dispute. Useful information on how to attempt to resolve disputes can be found in the government’s publication “Over the Garden Hedge”.

http://www.cornwall.gov.uk/default.aspx?page=2793

Oh dear Earthbound I do apologise as I am embarrassed for trying to answer you in the first place, as you now have virtually the same answer 4 times!