Thanks Von Lucky, the decals have had a couple of coats of Vallejo matt varnish without curing the issue. I wondered if it may have happened because I neglected to put a layer of varnish down before the transfer, I usually do & haven't had this problem prior to this.
I'll give the whole models coat of Windsor & Newton & see if that fixes it, if not I suppose I'll blend a little paint in around them & hope to obscure it during the weathering phase.