Thanks for your question.
If you are paying the mortgage repayments and 50% of the living expenses then it will be difficult for him to establish that he has an interest in the property. The Court would probably take the view this is no more than he would be paying more by way of rent in the event that he were living in other accomodation.
If there was no capital expenditure made by him when doing the DIY improvments then this would similarly not effect the situation.
You should consider making a cohabitation agreement with your partner, this is a deed signed by both parties which states whether or not each party is to have/accrue an interest in the house. It will also deal with other things such as property bought during the course of the cohabitation, debts, budgeting and payment of outgoings.
In order for it maximum liklehood of standing up in court you should both take independant legal advice on the document and be completely honest with each other about what you both earn.
Finally, and this is perhaps an obvious point, you should make a Will stating what you would like to happen to your estate upon death (ie. to kids). You may consider making a small provision to your partner in that Will to show that you did contemplate him and to provide some protection from the possibility of him making a claim under the Inheritance (Family & Dependents) Act.
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