Dear Peter
There is no reason to suppose the NBN holds comprehensive data on galls, in common with a great many other groups. You cannot therefore assume anything about the distribution of species from maps on the NBN. In Essex there are no maps available on galls other than those published in the past, most recently in the Essex Naturalist vol. 16 (1999) for Essex species of gall-inducing Cynipids on host plants other than Quercus by Jerry Bowdrey. If you want to know about the status of galls in Essex, you need to enquire with Jerry, the County Recorder, see county recorders.