I think there are 4 chiffchaffs, there's one that looks decent for a collybita, and three others that look eastern, like you say. At least one makes very good sibe noises and looks the part too. One that looks slightly less good sounds like it's doing a weak version of a standard collybita hweet, but the sonogram looks totally different.
I reckon 8 wintering chiffchaffs in Scotland is pretty remarkable.
I completely failed to see any Chiffchaffs, although I think others saw some.
Glad you got one in the end Hugh. I missed the rook....