A letter from the applicant is only optional and should really just be a contents letter introducing the package.
So, it can state:
- type of visa applied for and intended travel date (date you wish the visa to start from)
- an explanation of anything unusual about the documents, such as large deposits or withdrawals from the sponsor's bank account or reasons why something cannot be provided
- an explanation of any criminal convictions or bad UK immigration history (refused visas/entry, illegal overstaying or working in the UK) that the applicant might have
- a list of all the documents in the package
It should be short and to the point. No personal details about your relationship, as that goes in the sponsor letter.
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