All of these were printed hollow and I thought I had manged to drill and drain all the uncured resin inside. But the recent very hot weather has proved me wrong - there was still uncured resin inside and as it seeped away it blocked up the drain holes. It then expanded and forced its way out through the weakest points giving the impression of an internal explosion.
So back to the drawing board for these and attempt to find a better solution and replace the damage. I also have other models that have suffered collateral damage and have resin sprayed on them. Hopefully I can solve this by painting over with matt varnish.