Finally managed to dig out the certificate - and yes, Mum had moved it
Marriage solemnized in the Parish Church in the Parish of Ystradgynlais in the County of Brecon, April 20th 1839
David Lewis, full age, miner, residence Cwmgiedd, father Lewis Lewis, labourer
Margaret Leyshon, full age, [can't read occupation], residence Cwmgiedd, father Evan Leyshon, labourer
Found them in 1841 - Evan and Jennet Lyshon, both aged 55, and William aged 15, in Cwmgiedd Village, Ystradgynlais, HO107/1367/19. Both are born out of county. David and Margaret, both aged 20, are next door to them.