Since you referred to solicitors, I assume you're in Canada or some other Commonwealth country. We can't give you specific advice for other countries. Here are some options that are based on US law. They may or may not be viable solutions in your country. Try contacting a legal aid society, law school, or the Bar Association for a list of attorneys who do pro bono real estate work.
Sue gf to partition the house by sale. The court will order the sale & reimburse each of you for your share of the money you invested, then apportion the profit/loss 'equitably'. This will be expensive, require a solicitor, and take time.
Stop paying the mortgage & deed the house back to the bank. The bank will foreclose on gf, sell the house, and charge you & gf the loss. If there's a profit, the bank will divide it equally between you & you won't get your downpayment back. This will take time, damage your credit rating, & rack up foreclosure costs that you'll have to pay. You may be able to work something out with the bank to save your credit. This option is really only useful if you're going to take a loss anyway. The bank's mortgage department may also be able to help you explore your options-- banks hate foreclosing on houses.
Work a little slight of hand on gf: Get her to sign the house over to you without letting her know she's still on the hook for the mortgage. Then sell the house.
Get some sort of restraining order barring her from the house. Once she can't live there anyway, she's more likely to be willing to sell. I don't know how likely this is -- in the US you have to allege domestic violence or criminal activity in the house.
Sue her for not paying her part of the mortgage & request that agree to sell the house. You'll have to sue her in a court that can grant that particular form of relief.
You can't just change the locks because she's an owner and have every right to be there.
Your father might be willing to pay the solicitor to protect his loan to you. If he didn't record the loan on the house as a lien, he should do so immediately so he gets paid from the sale proceeds. Also, make sure the house is sold through an escrow company -- they'll make sure to pay him directly.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.
[This message has been edited by Tracey (edited June 27, 2000).]