I have a shared ownership with another person. they have 25% of the proceeds of sale and I have 75% of the proceeds of sale. as I paid down a deposit of £30.000 their share of the mortgage is 29 %. I have it in writing ( Declaration of Trust ) that they are responsible for their part of the mortgage but they moved out 11 years ago and stopped paying the mortgage. as they refuse to sign their share of the house over to me I cant move, sell or do anything. I feel stuck.
In the U.S. (and probably in the UK, too, given the common roots of their legal systems) you could resolve this in court with an "action to quiet title". Basically, you'd be looking for a court order that the property be sold and the proceeds divided appropriately. This probably isn't something you should do on your own; hire a lawyer. Another benefit of hiring a lawyer is that there may be other ways of resolving this.