FBR marriage certificate question

Hi!

I’m doing FBR through my grandmother.

I have her Birth Certificate and death certificate. However, I do not have her marriage certificate which is an issue because she did have a last name change upon marrying my grandfather.

They happened to get married in Bogotá, Colombia (random because they were both Irish citizens). It is not possible to get their 1958 marriage certificate from Columbia. My only idea is include a letter within the application documents declaring her last name change (like how you would if your address has changed).

If there are any more thoughts or ideas, please let me know.

Note - I would do FBR through my Irish grandfather because his last name does not change but we’ve unfortunately done the application and payment through my grandmother.