I have specific cover for customers work 'held in trust'. Never had to claim, but I would think that it the work happened to be damaged in any way it would depend on the circumstances on whether the insurance company would honour a claim. It's more to cover fires/theft/other things that you have taken reasonable steps to avoid.
Scenario 1. You have a painting on the workbench. Someone knocks a cup of coffee over it. Painting ruined... Legitimate Claim?
Scenario 2. You have a painting on the workbench. The people upstairs have a burst pipe which brings the ceiling down over your workbench. Painting ruined... Legitimate Claim?
Scenario 3. You have a painting on the workbench. A delivery man decides that is the perfect place to land a big parcel. Painting ruined... Legitimate Claim?
Scenario 5. You have a painting on the workbench. I looks a bit dirty so you try to clean it with Cillit Bang an a souring pad. Painting ruined... Legitimate Claim?
OK. You're the insurance assessor. Pick the bones out of that lot.
(i'm with the NFU btw.)