Changing current repayment morgage to buy to let???

Hi I was wondering how hard it would be for me to change my current repayment mortgage to a buy to let? My current morgage is for around £112000 and i have recently had the house valued at £135,000. My current mortgage is the wonderful Northern rock "together mortgage" so there is also a unsecured loan outstanding, £10,000.

Any help would be much appreciated. Im hoping to buy another house with my girlfriend who is a first time buyer so we are hoping that entitles us to use one of the new government backed schemes.....

Cheers