Hello mark, could be that not enough light is getting to the base, or have you trimmed to far in to the old wood, what about cats or dogs/foxes spraying, have they got any brown patches elsewhere if so then check for the conifer aphid, one solution to remedy these brown patches is to let some of the growth just above the patches unpruned for a while and skillfully bend and tie them in over the pathches, takes a little time buit will work, then remember when trimmimg/pruning to keep it sightly broader at the base and taper them up slightly so the light gets to the base, please see my blog re, the conifer aphid, hope this helps Julien.