Neglected beech hedges can outgrow any garden. In particular a beech hedge can get wider. However it can be brought back into shape quite simply. First, before cutting, it is a good idea to mulch your beech hedge well in the spring. Then, the following winter, you can really go to town.
Reduce its height by up to half if the hedge is too tall. Take it down a bit more than you need to so there is fresh growth on top for you to trim later.
Although you should not do so at the same time (i.e. the same winter), you can also reduce its width by up to half. Prune one side the first year and the other side the following winter. Take the branches on one side back to within 30cms (12") of the main trunks in the middle of the hedge (and very lightly trim the branches on the other side). Just do one side in one winter as you will cut out a large number of dormant growth buds, and these need to regenerate the following spring and summer.
If the first side grows back well, do the other one the next year (always in the winter). If recovery is slow leave the second side alone for a further 12 months.
For tall and fat hedges spread your remedial work out over three years. Reduce the height first as it shortens the sides lessening the work for subsequent years.
Water well in the year following pruning and remember your beech hedge would love a top dressing of Growmore in the spring.