Back by popular demand (and also timely from a personal standpoint) here is the Neighbourhood Support discussion on boundary fences:
"Disputes between neighbours over liability for the cost of a fence can end in resentment and make day-to-day life unpleasant. Some understanding of the law relating to these matters may help avoid problems that could damage neighbourly relations.
The Neighbourhood Support website provides excellent advice on this and other disputatious matters.
The Fencing Act 1978 provides a set of rules that govern financial obligations associated with a fence erected on a 'common boundary'. The simplest way to ensure contribution to the cost of building or repairing a fence is to get an agreement with your neighbour. To make sure there is no misunderstanding on the agreement, it may help to record it in writing and give both parties a signed copy.
Where agreement is not possible, the Fencing Act requires that neighbours contribute equally to the cost of an 'adequate fence', provided the procedures contained in the Act are correctly followed.