The older records of the Church were transferred to the custodianship of Birmingham City Archives, situated on the top of the Central Library.
To find items use their search engine, CalmView, with the search term "Agatha". That will bring up hundreds of matches many of which are in box DRO 43. Details of the contents of that box are available here.
Before visting the archive please contact them so that they have the records ready for you. The records may not leave the archive nor be photocopied.