Daughter's divorce

I am in my 80's. I have property valued at £700,00, which makes me sound rich, but I have only my state pension to live on - which is adequate.
My daughter is in the middle of a rather nasty divorce, so I approached the solicitor who drew up my will and asked him to make an addition to it, saying that should I die before the divorce was finalised, her share of my estate should be diverted to her children. I don't want her soon to be ex-husband to get his hands on it.
He insists I make a completely new will and power attorney - which will cost me £100s which I do not have.
Why can't he just write out my wishes and make an addendum (would it be called a codicil?)
What is my legal position?
Don't say go to another solicitor - I really don't have the money - its just not there! Do I get a bank loan?
My friend says I should write a note out myself and get my signature witnessed - would that be legal?