tis not from Dylan's play, Under Milk Wood.  That work is on the web and a 
search reveals no carpenter in it.  The question as posed was

long long time ago, I had a, (since lost), two cassette set
called "The Rime of The Ancient Mariner
And Other Poems Read by Richard Burton."
In it there was a poem where the scene is
the ruins of some old building at night. The speaker
relates how walking and standing there
he encounters another man who speaks no words.
The man puts his hand on a large fallen wooden pillar
and speaker remarks to him: "It's the kingpost."
As the man keeps his hand on the post, it slowly rises
up and sets again perfectly vertically in its hole.
The astonished speaker cries out:
"Who are you?!!"
to which the man softly responds:
"I was a carpenter."

As I thought about it, Priest and Peasant,
(by R. S. Thomas), sounds like a possibility,
it denotes two characters, and though I don't
recall maybe the ruins are that of a church.
Remember we were working with a known list
of possibilities:

BY Samuel Taylor Coleridge:

Frost at midnight

By Dylan Thomas:

Fern Hill
The Hunchback in the Park
A Winter's Tale
Deaths and Entrances
Before I knocked
Lament
Lie still, sleep becalmed
Poem in October
And death shall have no dominion
Elegy(for his father)
The Rev. Eli Jenkins from Under Milk Wood

By Christopher Fry:

lines from The Boy with a Cart

By Bishop Henry King:

lines from Exequy on His Wife

By R. S. Thomas:

Priest and Peasant
The Lonely Farmer

By John Betjeman:

In Memoriam Walter Ramsden

By John Donne:

A Hymn to God the Father
At the Round earth's imagined corners

By Francis Thompson:

The Hound of Heaven

*

Most likely it is from RS Thomson, Fry or Betjeman.  The others have been 
found on the web.

These are not exactly common on this side of the pond, so if one of your 
volumes contains any of those poems, if you cousins could do a search.

Smiles and thanks.

Gary


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