ILR letters proving joint residence

My wife is applying for ILR on the 5-year spouse route.

It is required to provide 6 jointly addressed letters/documents to prove cohabitation (or 12 individual ones). I am struggling to find specific guidance on these

We don't have large numbers of different ones

a) Do they all have to be from different agencies ? (or could I submit two consecutive annual joint council tax bills)

b) Do they have to be from government agencies or does anyone know if more 'informal letters' are acceptable? eg we have a letter addressed to us both showing joint National Trust membership.

Thank you for your help
Steve

There is no sepcific evidence. Any evidence can be used
indefinite-leave-to-remain/cohabitation ... l#p1813810