Odd Chiffchaff at Bosherston

An interesting Chiffchaff was at Bosherston pools this morning. Views were brief and frustrating, but the was a yellow wash to the throat, yellow supercilium, although not very bright, and paler brown legs. It would disappear for long periods but would often give its self up with a loud, upslurred "sueep" call, it never once called like a common Chiffchaff. Unfortunately,  I couldn't get it to sing, despite trying. Iberian kind of fits the call, but not fully, and with no song I'm not sure what it could be. The bird was near the grassy bridge at Bosherston pools. Walk over the bridge and along the path for about 100m towards Broad Haven south beach and look into the trees along the path, particularly where the woodland comes to an end.