I finally returned to Crewkerne Sewage Treatment Works yesterday (Friday 26th Jan) to look through the Chiffchaffs. There appeared to be two candidate Siberian Chiffchaffs - one a little more grey-and-white and the other more brown-and-buff (not much in it, but they appeared alongside one another allowing comparison). At no point did I hear the birds call and so I played some Sibe Chiffy song from a Bluetooth speaker I lodged in a tree. That’s when I realised there were potentially two birds as both popped up in the tree. There was no wing shivering or calling/singing back but the birds were very lively. The browner bird did not show well and soon disappeared. None of the obviously olive/green/yellow Chiffchaffs appeared to react (there were several singing around the area anyway). The greyer bird appeared a little larger/more robust, but I think this may have been an illusion created by a stronger head pattern with bold supercilia. As a result of my observations yesterday I am certain that there is one, and most probably two, Siberian Chiffchaffs present.

There were plenty of other birds present yesterday in the sunshine but the warblers were not coming down onto the filter beds as they had previously so were more difficult to count:
Chiffchaff - about 25? Several were singing over quite a wide area and I would imagine there were many more than this.
Goldcrest - c10
Bullfinch - 1
Sparrowhawk - 1 doing regular circuits and make all birds very flighty.