I think the following combination of features identifies this as Alstrom's Warbler:
Unbroken eye-ring
Black lateral crown stripes which only reach to above the eye
Pale (rather than blue-grey median crown stripe
Limited grey below lateral crown stripes
Dull underparts
Darker wash to sides of breast
Very indistinct wing bars
Very limited white in the outer tail feathers which does not inlcude a triangle of white extending towards the tail base
Date is also good, with Altrom's being an early migrant (already migrating through Thailand in mid-Sept)
