A general question about a specific request from co free holder

We live in a ground floor maisonette, having purchased the property in early 2021. Our co freeholders upstairs recently put their flat on market and is in the process of selling it . They have approached us with a request to give a specific letter as requested by the buyer's solicitor that as co freeholders we retrospectively consent to the erection of an L shaped rear roof extension with balcony of roof lights etc and so on. This work was carried out in 2014 when we were not living here. They are very specific about the wording of the letter and although we gave them a letter saying we moved in early Jan 2021 and that we are aware and consent to the existence of the loft extension, they are not agreeing to this. They are very specific about the wording and want us to sign the letter as co freeholders which is making me very nervous. I have never heard about a similar request considering that we moved in later and was not part of any consents or aware of how the work was conducted. Any idea why the buyer's solicitor need such a letter and any legal implications of signing such a letter.