Answer: What a wonderful project! The ancient practice of pleaching is closely related to espalier in the sense that it is an extreme form of "training" and needs to be done on an ongoing basis. Essentially, pleaching involves uniting the branches of shrubs, vines, etc. by plaiting, weaving, or braiding them together.
The timing of the leader pruning depends on the ultimate shape you have in mind for the tunnel. If you are expecting to have widely spaced columns holding up the sides of your tunnel, then wait. If you are expecting more of a series of candelabras along the sides of your tunnel then do it earlier. The secret is in watching the individual plants on a regular basis so as to end up with a symmetrical result.
Laburnums need evenly moist soil yet well drained soil and, as you have witnessed, are very susceptible to drought, even more so as young trees that aren't yet established. Excessive heat can also affect it adversely -- this is not always an easy tree to grow and it is not a fast grower even under the best of conditions. Good luck to you!
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