Exochorda wilting - no idea why ...

It seems more likely to be drought. If you've had recent rain then the surface may be wet but it could be dry further down. Give it a bucket of water - it can't harm it.

You say the ground is heavy clay. Did you plant it straight into the clay or did you make a big hole first and fill it with compost etc.? Planting directly into clay is not good as it becomes difficult for the roots to establish. Clay is very nutritious but it will struggle if there's no softer material for the roots to establish itself into first. If it was landscaped I would guess they did the job properly but you can still ask them to give you advice on what the problem might be.

If it came in as a standard then it would have come in a pot/bucket of soil or in a root ball. A hole should have been dug at least twice the size of the pot or root ball and filled with good soil and compost.