Presumably another Siberian Chiffchaff: the one inflected note it gave on release wasn't the 'usual' plaintive call although it does conform closely to other recordings of autumn juveniles:
Overnight rain that lingered on into the morning eventually cleared to reveal some sea passage and a small arrival of grounded migrants. The sea was well worth attention, with 250 Kittiwakes, 20 Teal, 2 each of Arctic Skua and Great Skua, and a single Red-throated Diver through off the Bill. New thrushes were evident on the land, with particular concentrations of 30 Blackbirds in and around the Obs and 40 Redwings, 25 Blackbirds and a Mistle Thrush in the Foundry Close/Easton Fire Station area; also new were 4 Blackcaps, a Siberian Chiffchaff, a Chiffchaff and a Brambling at the Obs and 200 Starlings in off the sea at the Bill; the Siberian Lesser Whitethroat also remained at the Obs. Goosanders have been cropping up from time to time at Ferrybridge, with 3 through there today and 4 nearby in Portland Harbour; 2040 Dark-bellied Brent Geese, 24 Bar-tailed Godwits, 3 Pale-bellied Brents, 2 Knot and the Black Brant were also at Ferrybridge.