Upon returning home I've analysed the song on the videos and can say with some certainty that this bird was a Western Bonelli's Warbler.
You can hear the bird singing after 3 seconds, 11 seconds and 30 seconds in the video below.
I've made reference to an article on Bonelli's Warbler on the Sound Approach website for this analysis: http://soundapproach.co.uk/sound-approach-chip-in-on-id-of-britains-sixth-eastern-bonellis-warbler/
and Dutch Birding: http://www.dutchbirding.nl/journal.php?id=235
The spectrographs are made using Audacity: http://sourceforge.net/projects/audacity/
The tell-tale signs in the two spectrographs below are:
1. Maximum frequency - Western BW has a maximum frequency of 7200 Hz (as seen here), compared to 6300 Hz in Eastern BW.
2. Average number of notes per trill - both spectrographs show 9 clear notes. Western BW has gives between 7 - 11 trills, whereas Eastern BW gives between 11 and 19 notes.
3. Trill length - both trills last for 0.65 seconds. Western BW has a song-length of 0.53-1.24 seconds whilst Eastern BW has a song-length of 0.71-1.70 seconds.
4. Shape elements - we can see 'tick-shaped' marks on the spectrograph. Sonograph notes of Western BW are described as "upright-V" whilst those of Eastern are described as "inverted V(^) or mirrored N (?)"
So, the spectrographs clearly show that this bird was Norfolk's 13th record of Western Bonelli's Warbler.