The letter literally just introduces the contents of the package - you don't need to talk about yourself or your relationship.
You can include:
- the type of visa you are applying for
- the date you wish the visa to be made valid from (the intended travel date on your application)
- an explanation of anything unusual about your documents (i.e. if you don't have an original of something, or there are large transactions on your bank statements)
- an explanation of any criminal convictions you have, or any bad UK immigration history (refused entries/visas for the UK, illegal overstaying/working in the UK etc.)
- a list of documents in the package
So an example letter, if you have nothing to explain in regards to documents/convictions/immigration history might be:
To The Entry Clearance Officer,
Please find enclosed my application and supporting documents for a settlement visa to join my spouse/partner in the UK. I would like my visa to be made valid from DD/MM/YYYY.
The documents included in this package are:
-
-
-
etc.
Signed,
Your Name.