Odd Sounding Chiffchaff

I heard a warbler singing at Shawell A5 Lagoons yesterday, which was obviously a Chiffchaff sp. but the song seemed wrong for what I would expect from an ordinary British breeding Chiffchaff. There are elements of the song that sound ok, but much of the middle section that you can hear in the video below seems very wrong for collybita. When I listen to it with ear phones on I can hear two parts that end with a faint Reed or Sedge Warbler like bit. I saw it briefly and it was obviously a Chiffchaff complete with a pumping tail, but I noted a yellow wash to its breast. The yellow wash to its breast made me think of Iberian Chiffchaff but the common Cetti's and Wood Warbler elements of the song were missing.

Below is a sonogram made from the song on the video. I'm afraid I'm no expert on sonograms, but maybe one of you can help me. It doesn't match anything I've seen so far?

Any thoughts?