As time has passed and with the publication of a new paper by Svensson in this month's BB, there should be no doubt that this was an Eastern Subalpine Warbler. You can see Martin Garner's post here for further details and images.
At the same time as the Yell individual, another male was found by Martin Kitching of Northern Experience Wildlife Tours while on a busman's holiday birding in Druridge Bay on 4th October. It remained until the following day. Being just down the road, I caught up with this male on its first afternoon and enjoyed some brief views as it moved around the blackthorn and roses. Aware of the potential three-way split in the, perhaps not too distant, future I was keen to see if we could 'do' this one to sub-specific identification. There was much discussion of identification features while on site with Martin and one or two other birders in attendance.
Both Martin and I gained an impression of a bird with quite whitish underparts with a pinkish tinge around the upper breast and flanks. We also thought the white sub-moustachial stripe looked quite broad. It didn't call during the hour or so I was there. Confusingly, some of the images that began to emerge later that evening seemed to show a richly-coloured individual contrary to our observations in the field. It's fair to say this prompted a fair bit of head-scratching and re-reading of existing literature. Snippets of the yet unpublished Svenson paper suggested tail pattern could be all-important in determining the sub-species (and the possible future armchair county tick!).
Martin was able to get back the following day and with a great deal of diligence captured a flight shot that highlighted the all-important tail-pattern feature described by Svensson as part of his proposed split of cantillans into iberiae (a subspecies of Western Subalpine Warbler S. inornata) and cantillans sensu stricto (now the nominate subspecies of Eastern Subalpine Warbler) thus "In central and south Italy, adults and many first-years have a narrow white wedge on the inner web of the penultimate retrix, whereas the Franco-Iberian population has a small square white tip to this feather, not a narrow wedge" (Svensson 2013. A taxonomic revision of the Subalpine Warbler Sylvia cantillans. Bull BOC 133(3): 240–248).
Eastern Subalpine Warbler, Druridge Bay, © Martin Kitching
As can be seen from Martin's image above the 'penultimate retrix' (or tail feather) on the Northumberland individual clearly shows a narrow white wedge extending up the lower half of the inner web. Combined with our observations in the field on the extent of underpart colour and the thickness of the sub-moustachial stripe, this would suggest that if/when the proposed split is adopted this individual would fit neatly into the 'Eastern' side.
Currently Northumberland has only one record of Eastern Subalpine Warbler (S. c. albistriata): an individual trapped in November 1963 at Hauxley. This could, of course, be subject to change post any split/review.