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When the will was made the mother said to my other half his siblings "would have to have something" if he were to continue living in the house after her death, which he agreed. He gave her a figure he could raise & that's in the will to be split between the 3 siblings within a year after her death. It seems now that it's being contested on the assumption that that amount was related to the value of the house at the time the will was made. That was never the case & the will doesn't mention that. The brother's saying that as the will was made over 10 years ago, the lump sum to be split should be raised to reflect the value of the house now. He's also wrongly assumed that money would be raised by mortgaging the house. This is why notes would be handy as that would all probably have been included in them.
Several solicitors have given advice now & pretty much all of them have said keeping any notes indefinitely in the event of this kind of situation is good practice. There's no need to destroy them when there's a very good chance they could be useful - the older brother said from the day the will was written he'd be contesting it.