Set(o) other purposes letter

[Sparkie_bran]

Hi, I hope you can help.
My wife is on discretionary leave to remain outside of the immigration rules (private life).
Do you have any pointers as what to put in the letter we have to write to explain why she is using set (o)?
I was also planning to include evidence of cohabitation and my passport, just to make sure theres no reason to refuse. ( I'm a UK national)