Q.Virginia creeper has completely defoliated….
My Virginia creeper has completely defoliated, and I cannot see an obvious reason why. There is no sign of pests or the roots having been disturbed. The plant is about 7 years old and is normally completely covering the fence in July. In late June/early July it started to look like it was wilting. I watered it even thought I doubted that was the cause, as we had had plenty of rain. The leaves continued to wilt, they reddened slightly and then all fell off! I have never seen a completely bare VC in July! Anybody got any ideas?

Creeping Virginia can suffer from pests infestations and fungus issues.
Inspect the vines for any other signs.
I would treat the plant with Neem Oil. Neem Oil works as both an insecticide and fungicide.
Make sure the soil is draining and the rain you mention has not caused root rot.
The other possibility is herbicide damage. Is it possible it could have been sprayed or damaged by herbicide overspray?
I would clean up and dispose of any damaged or dropped plant material.
In most cases of Creeping Virginia, it is very hardy and will recover.
https://www.gardeningknowhow.com/ornamental/vines/virginia-creeper/virginia-creeper-maintenance.htm