I will list here the resources that may be of use: I will update from time to time.
Advice Guide, the CAB online portal is a marvellous resource on family matters including aspects of family life that people find most troubling such as what happens if your partner dies and there is no Will.
A gem of clear web design, Child Maintenance Options, has maintenance and expenditure calculators as well as a precedent for a private child maintenance agreement.
The Child Support Agency site contains, amongst other things, a useful calculator based on the present child support assessment model plus information on how to involve the CSA if you cannot broker a private child maintenance agreement with your partner.
DirectGov offers general but comprehensive guidance on all matters relating to divorce and separation.
The Money Advice Service offers budgeting tools and a calculator for working out potential scenarios for asset splitting.
The Pensions Advisory Service can give you information about anything to do with your pension, including occupational pensions, stakeholder pensions and personal pensions.
Legal expertise is not the whole answer in many cases. The Life Coach Directory is a useful site for finding a life coach near you who may be able to assist in the self-awareness needed to make positive decisions about your future.
Cafcass have some good practical guides for separating parents and I like their resources for young people and children.