According to the 2003 report THE place in the county for this skulking little warbler is Thatcham marsh where 17 were noted on April 6th.
The birds are surprisingly mobile; for instance "The Birds of Berkshire" refers to a bird ringed in Thatcham in June 1989 and re-trapped at a different location in July 1990.
Most reports still come from the Kennet valley but there is an increasing number of records from other sites. The Tetrad survey (1987-1989) recorded Cetti's in eight tetrads (2%) including the Lambourn valley and the Blackwater. The bird has spread still further. The excellent "Berksbirds" website has recent reports of winter birds at Dinton, Bray and Moatlands.
The warbler's preferred habitat seems to be Phragmites reedbeds bordered with either willow, alder or other scrub.
"The Birds of Berkshire" reports it as breeding in Berkshire in 1971 and claims it as the first inland record for the UK. Records of breeding are difficult because of its habits & habitat but there are almost certainly more than the two reported in the 2003 report. Cettia cetti is on the green list with an estimated 550 pairs breeding in the UK
Cettia cetti in The Kennet valley