No Lenders actively accept self-certification applications from first-time buyers on an equity release basis - so you will need to look for a mortgage with proved income.
If you can get written confirmation from your employer that your contract will be made permanent from September 07, you will not have any issues. However, if this cannot be confirmed, it would be a case of understanding your full circumstances and talking the case through with individual Lenders to get agreement slightly outside policy. Most Lenders are comfortable Lending to applicants on fixed term contracts as that is becoming more common place. However, they look for a stable track record of renewed contracts and usually look for a minimum of 12 months continuity of employment that you cannot prove yet. So initially try and find out if your Employer is willing to confirm that your contract will be permanent from September. Failing that a good place to start would be your Bank, if they offer mortgage facilities.