Well, I’ve heard back from Santa Cruz and they are replacing the frame under warranty. Santa Cruz US said the cause was a slight carbon delamination in that area. They’ve not seen it happen there before but there is no generic problem and said you do get some random issues with bikes.
Have to say, I’m very impressed with customer service from Jungle and Santa Cruz. Also very relived it’s been sorted, as not sure what I’d have done if they’d said it was paint, as I didn’t believe it was.