One of the characteristics of "The Ghost at the Old Lychgate, St Mary, Berkhamsted" is the length of the lines. Long lines have been used to control development of the narrative, the movement of the poetry, the speed of reading and to give impact at certain points.
Long lines tend to add a prose-like nature to a poem and prose is often used to develop a narrative and tell a story. There is an implicit narrative to this poem and the use of long lines allows the reader to develop the "back story" without making it explicit.
Using a long line length to control the speed of the poem is important here. The time it takes to read each line, or each narrative item of importance in the poem, slows the reader. This gives time for the item of importance contained in each line, perhaps simple description, implication of a back story, the properties such as the speed of occurance of an event or setting, to be realised and appreciated. This allows for a more meditative and considered reading of the poem to take place.
This poem is elegaic. Long lines allow the poem time to drift, rather than the "bang-bang-bang" impact of short lines. In this poem, the story uses a dreamy drifting progression to build an atmosphere. There is a development of innuendo and implied events to allow the reader to anticipate or even predice the ending, which contains the unusual event of the groom being jilted, rather than the bride.
The long lines also reflect techniques often used in ghost stories to build and maintain suspense, where little seems to happen but much is implied. This builds up suspense which is often released by a sudden shock. This poem does not have a shock at the end, but a twist which I hope is unexpected.