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As you are separated and not divorced, you can ask for a separation agreement to be signed which would give you protection and if you did divorce you could refer the court to the terms when an a financial is applied for. A separation agreement template can be downloaded and tailored at http://www.lawdepot.com
Your contributions to the household bills and the mortgage payments would be taken in to account. You have an 18 year old son - the court would allow you to stay in the home until he reaches 18 but as he is already 18, there is a chance your husband might ask for a sale of the former matrimonial home if he is awarded a share.
In terms of what you are entitled to, there are numerous factors the court considers such as your husband's earnings compared to yours and availability of assets. The court will consider Section 25 of the Matrimonial Causes Act 1973 when it assesses a financial settlement. Therefore your ages, length of marriage, any children under 18, your earnings and earning potential (same for your ex), salaries, the assets including the former matrimonial home and pensions if applicable, any debts, financial and non-financial contributions to the marriage, your standard of living enjoyed before the marriage breakdown, your financial needs in future and your housing needs, and so on.
The starting point is 50% of assets (including the home) but if there is a disparity in earnings, you can ask for more than 50% of the former matrimonial home and you can claim a share of the pension too.
It would be worth speaking with him to see if he would agree to any split. If not then you would need to issue financial remedy proceedings once you have either applied for a judicial separation or a divorce, so you ask the court to become involved in a financial settlement. To do this you would need to complete and send two completed copies of Form A (one copy is attached) to your local family court and pay a fee of £255 payable to HMCTS. If you are on low income or having low savings, you may be able to ask the court for a fee remission – you can check your eligibility here: https://www.gov.uk/get-help-with-court-fees
The court will then list a first appointment and give directions for a financial dispute resolution hearing (FDR) where the judge will give their opinion on a likely settlement - which usually prompts a settlement at this stage. If the case does not settle, further directions will be given and a final hearing may take place if neither of you can agree to settlement terms.
To apply for a judicial separation the following site has details of how to apply: https://www.gov.uk/legal-separation
To apply for a divorce: https://www.gov.uk/divorce/file-for-divorce
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