What are my legal rights

My father-in-law, upon death, left in his Will 50% each to both his daughter and his grandson and both were appointed as trustees. His daughter, who was my wife, has since died and any share coming from her fathers Estate should be paid to her Estate. As I am the only beneficiary of her Will I should receive that 50% from her father's Estate.

I do however have a problem with the grandson because he is not prepared to let me finish and process a claim with the solicitors, for Retrospective Healthcare Funding in respect of his deceased granddad's Claim. He won't even take the case on himself. The case is 70% of the way through after all my hard work and is nearing completion after seven years of my working with solicitors. As we are nearing the point where funds will be claimed from the NHS he will not allow me to conclude the matter to enable the funds to be paid to my Father-in-law's Estate. I have dealt with this matter from day one but the solicitors have now told me that due to his stubbornness, I should now seek independent advice.

As sole living trustee he is of course liable to distribute any funds himself, in accordance with the Will but he is even refusing to do that. There will be no compensation paid until the solicitor completes the claim, which he cant until the grandson decides how he will deal with this. He has decide how to move this forward, ie. Take the case on himself or give the authorisation to me to complete the claim with the solicitors. As yet he wont do either.

Surely he can't refuse to do this? The solicitors dealing with this case have told me that it looks favourable in the seeking of compensation.

Please may I ask for your advice and what rights do I have?