"how much a solicitor should cost if i want to get the name changed on the mortgage to just my own."
Its not just changing the name on the mortgage - if it was just a tippex job, you could do it yourself. Its about buying your gf out so that her name can be removed from the title and remortgaging or topping up the mortgage and joining lender in the Mortgage Deed for you to take over all the responsibility of the mortgage.........
There is also the issue ( almost inevitable we find) that the parties are barely speaking, hate each other and despite the obvious advantages of completing the transaction quickly, are determined to outdo each other in nastiness and lack of co-operation.
mf