I have telephoned Llandod this morning & they hold the marriage register for Llan-nant-bran from 1813 onwards & the previous register is lost. However the bishop's transcripts frm 1719 -1809 survive but are held at the National Library of Wales in Aberystwyth. Bishop's transcripts are patchy but it might be worth e-mailing them with your request as they are quite helpful & it would be a very quick lookup. There address is holi[at]llgc.org.uk , but before you do, are you sure you are talking about the correct parish? There are a lot of Llanfihangel-nant-somethings in Wales! I notice you have a year & parish but no actual date of marriage so assume you have the detail from the IGI submitted by a member? These entries can be way off mark, I had one which said a marriage had taken place in Scotland which had actually taken place in Birmingham!
If you have no joy with Aberyswyth, or want further help, let me know as I will try to help if I can. By the way there would be no chance of any registers for L-N-B in Crickhowell, but if you want photos of that area or L-N-B, I can help out. so feel free to ask.
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