Thanks for your question.
If your partner is not going to pay the mortgage directly, will not pay over and above the amount he would otherwise pay in rent somewhere else and does not spent capital monies on making improvements on the property then he would not be able to claim against the property.
To evidence this intention 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. A solicitor could draft, advise and arrange execution of such an agreement for £300+VAT approx.
In order for it maximum likelihood of standing up in court you should both take independent legal advice on the document and be completely honest with each other about what you both earn.
If he is to occupy the property then you will need to speak to your mortgage company about what they require so that you do not breach a term of your mortgage with them., They will likely require that your partner executes a deed of postponement stating that he has not and will not acquire any interest in the property and will agree to vacate it in the event that it is sold. He will have to take this to a solicitor to execute it and it will cost in the region of £70.00 +VAT.
If this is useful please kindly click accept so that I may be rewarded for my time. It will be gratefully received and you will be free to ask follow up questions.
Edited by Thomas on 10/5/2010 at 12:05 PM EST