My partner did fill out and sign the SU07 sponsorship undertaking form - is this not equivalent to the letter?
Unfortunately not, the SU07 form is not required for an unmarried partner visa and is unlikely to be considered. It is only used for exceptional circumstances visas, and in those cases is required in ADDITION to the sponsor letter.
For example, for elderly dependant relative and adult dependant relative visas where the applicant is completely physically and financially dependant on the sponsor... i.e. they cannot walk, cannot dress or feed themselves, they cannot work and they have no one to take care of them in their home country, which means they HAVE to move to the UK to be cared for by the sponsor.
The sponsor letter usually takes the form of:
- their ability to be a sponsor (UK citizenship)
- how they meet the financial requirement and under what Category they are applying
- where you will live together in the UK
- a short history of your relationship
- a list of documents the sponsor is providing for the application to prove they meet the requirements