In all honesty, this is a very difficult bird to assess - it is a male in extremely fresh plumage with what appears to be fresh primaries and therefore most likely an adult bird rather than a first-summer. As others have already expressed in litt, this bird has heavily contrasting underparts, with the rich orange-red chin, throat and breast sharply demarcated mid-breast and paling into whiter lower underparts. On a prolonged view however, these lower underparts (belly, flanks, vent) were seen to be quite sullied with an overall light pinkish or light fawn wash - and certainly NOT the gleaming white of a typical adult Eastern Subalpine (albistriata). However, more prominent still, was the white sub-moustachial stripe, remarkably broad and flanging - a definite feature of Eastern.
The entire upperparts were greyish with the head particularly blue-grey and the mantle and scapulars darker and infiltrated by more brown feathering. The lesser and median coverts were uniform grey, the rest of the upperwing being similarly coloured, with some pale fringing to the inner primaries, secondaries, greater coverts and tertials. The eye-ring was largely pinkish-orange.
So, in summary, a bird with somewhat mixed characters, which could be either/or Western/Eastern. I did not hear it call today but cantillans (Western) has a Lesser Whitethroat-like low-pitched, monosyllabic, single ''tukkk'' contact note whereas albistriata (Eastern) has a louder, often repeated ''trekk-trekk'' note (Moltoni's Subalpine of course having a very distinct Wren-like churring scolding call-note). Anyone hearing it could assist with the identification.
My gut feeling is that it is a male Western Subalpine Warbler but in any event it is extremely similar to a male Subalpine Warbler that was present on the John Weston Reserve at The Naze from 15 August until 2 September 1993 (see photograph in Birding World 6: 354), although this individual was more uniformly coloured on the underparts.
The hedgerow today also held several Lesser Whitethroats, Common Whitethroats and the odd Willow Warbler, whilst when the wind picked up from the south, Common Swift passage seriously took place (with some 64 birds flying south in just half an hour). Hirundines too were on the move, with 150 or so Barn Swallows heading south and a much smaller passage of both Sand and House Martins.
On the neighbouring Holland Haven scrape, there was an excellent selection of waders present and visible from the hide: highlight was the 3 juvenile WOOD SANDPIPERS, along with 4 Pied Avocets, 9 juvenile Ruff, 7 Common Sandpipers, 1 Common Greenshank, 2 Dunlin and 29 Icelandic Black-tailed Godwits.