How can I find out if my nan actually wrote her will or if it was fraudulent written by my uncle?

My nan died in 2003 and apparently wrote a will, my uncle was left her home but it was actually my dads as he paid my uncle for the house in 2000. My nan knew of this happening, when my nan went into a care home my dad then moved into the house which he had completely renovated. If my nan actually wrote the will then she never changed it to say the house was my dads and in 2008 my uncle registered the will with the high court and then registered the house in his name with the land registry which he should not have done as it wasn't his to do, so no one knew at the time he done this as we never thought my nan had written a will. I don't understand why he waited 5 years to do this if he had a will of my nans. My dad died in 2016 and I was granted probate. My son has been living in the house for the last few months as it had been empty befor then. All of a sudden out of the blue I got a letter saying I had to go to court because I was trespassing on his property and he wants me gone. I can not believe this is happening and don't know what I can do about it as I have no proof that my dad paid Kevin for the house as most of the payments were made in cash. I do know that some cheques were written to him at the time. Can anyone tell me if there is anything I can do at court?