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Gut feeling from the picture is that it is a Chiffchaff, but I could not really back that up. If you heard the song, that would certainly differentiate it from Chiffchaff. Garden Warblers are probably just starting to arrive back from Western Africa(but none on my site yet, where they are common breeders).
Any more details to provide?
My Flickr photos...
It was high pitched and a single note sung repeatedly.
Almost certainly a Chiffchaff, then.
Lat/Lng: 54.92528, -1.72866
OS grid ref: NZ174589