Dinorwic is surely core range, rather than fringes?
One thing I've noticed with Northumbs is that they disappeared earlier from sites with heavy people, and particularly dog-walker pressure; the remaining likely breeding site isn't in their original core area in the county, but is noticeably less infested with dog walkers.
But otherwise the clear trend is for the numbers to be doing best in Scotland, so climate warming may well be important too. Milder winters in particular may allow increased competition for food resources from resident species.
What was on my mind was that in my area, it seems to me like they are retreating from the woodlands around the fringes of their local range, by which I meant the ones which are lower in altitude. So for Coed Dinorwic, in the past I've had them at 350 feet ASL, whereas now there are none. There were other "lower" woods we surveyed in the areas round Maentwrog and downstream from Nantmor which also seemed to lack Wood Warblers in prime habitat.
2 yrs ago, I was surveying the large NT Ysbyty Ifan estate, which is on quite a high plateau - decent areas around 900 feet and there were really good numbers of Wood Warblers. While doing this, I was asking 2 friends doing nestbox schemes several miles further downstream if there were the usual numbers of Wood Warblers in their woods, and it seems there weren't. There is, of course, always the danger of confirmation bias.
The points about disturbance and mild winters are interesting ones. They can be extremely confiding birds, singing within close proximity, sometimes coming close to check me out, then carrying on their business.