Pogle 1962,
I have a copy of the 1836 marriage and can send you an image if you let me have your e.mail address via PM.
Checked Glam FHS transcriptions of number of Parishes around Llangyfellach/Clydach for a possible birth for Daniel/David (including Ystradgynlais)- but no luck. Could be Daniel was born in Carmarthenshire/Breconshire - this area can be a bit fluid!!
The Glamorgan FHS transcriptions has 2 entries for the marriage ie.
Coslett, Daniel, of this Parish and Tamplin, Mary of this Parish, Llangyfellach 2 Jan 1836 (P,B)
Coslett, David of this Parish and Tamplin, Mary of this Parish, Llangyfellach 2 Jan 1836 (P, B, T)
The codes are P (with consent), B (Banns), T (Bishop's Transcripts)
The FHS Transcriptions also have 2 baptismal entries for the first child
Coslett, William, father Daniel Forgeman, mother Mary, bapt 15 May 1836, address Clydach
Coslett, William, father David Forgeman, mother Mary, bapt 15 May 1836, address Clydach
regards