cheapest way to get a solicitors letter drawn up?

my grandparents died some years ago and my dads twin brother and his family took it upon themselves to take all of their possesions for themselves , leaving my father with nothing and only £500 of the £1000 that was left for him. the possesions dont matter or the money, we have written asking for copies of family photos and had no response., would like a legal letter drawn up hoping this would prompt them. can anybody tell me the cheapest way here in the uk. by the way they left no will