I fear the clay soil it too wet / dense for them. Would it help to dig them up & replant them after putting drainage in?
You may want to try amending the soil if you're going to dig them up. Here's an article with tips:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm
This article describes how to dig up a mature tree: https://www.gardeningknowhow.com/ornamental/trees/tgen/moving-mature-trees.htm
It is honey coloured and of jelly constituency. I thought at first it was honey fungus but there's no sign of any fungi around
I would take the pictures (take a few that are even more close; a few above the area where the substance occurs) and samples to a local agriculture extension service or a local plant nursery to get a diagnosis. The pictures remind me of bacterial canker (also called “gummosis”, “blossom blast”, “dieback”, “spur blight” or “twig blight”) but it is best diagnosed in person.
https://homeguides.sfgate.com/cherry-tree-gummosis-39725.html
Do they suffer from winter burn? Are they a messy shrub?
Yes, they can be grown in zone 5, but you are right on the cusp in terms of hardiness zones.
Yes, they can suffer from winter burn; their roots are shallow. And they can be messy!
Here's a link to articles that may be helpful:
https://www.gardeningknowhow.com/ornamental/shrubs/cherry-laurel
Hi I can't get in to dig where my bush is old and thinning due to the hedge itself and the roots. I want to buy 30 small Cherry Laurel plants and cut the bottom out the pots and force them in to stand at the bottom middle of the hedge. Will the roots grow and find their way down and in a few years take the pots away. The Laurel can then grow up and through my hedge as it is looking bare and holey.
If I am picturing this correctly, you could cut out the bottom of the pots and set them inside the hedge, but after they grow for a year or two, you would have a terrible time trying to remove the pots. Plus, when you did, you would have exposed roots from the side of the pots.
A better option would be to remove the current old hedge and start anew with the Cherry Laurels.
I live in Union NJ The plants are 4-5 ft tall Thank you!
It could be some winter desiccation. You are in hardiness zone 6b and cherry laurels are hardy to zone 5. Those two in the center may be subjected to more drying winds than the other shrubs. Here is more:
https://www.gardeningknowhow.com/plant-problems/environmental/desiccation-in-plants.htm
https://www.gardeningknowhow.com/ornamental/shrubs/cherry-laurel/growing-cherry-laurels.htm