Wills and Letter Of Wishes specifically

Asking please if anyone has knowledge or experience specifically relating to a Letter Of Wishes to accompany a will.

I currently have a simple will (written using a relatives legal template) and properly witnessed - as an interim measure just for now.

My dilemma, and the reason for not consulting a solicitor yet, is due to my constantly shifting thoughts in respect of distribution of the final third of my estate.

I would like to leave details and beneficiaries for the final third - but cover this separately in my Letter of Wishes. (E.g one third of my estate to my family, one third to my husbands family, and one third to the specific charities and persons outlined separately in my Letter of Wishes). I would then be able to adjust and update the final third myself periodically.

The reason I have not yet been to a solicitor to have a will written is that my wishes have changed frequently over the last decade, sometimes even changing monthly, (for example a new charity to include, or a friends behaviour might unexpectedly be unethical, or a relative might behave unkindly - and as a result I reconsider and wish to remove, reduce or change the way they might benefit from this final third of my will, (which could easily be updated by me as and when required).

Does anyone know if it is permitted, legal and appropriate to include beneficiaries in a separate Letter of Wishes in this way?