A phyllosc was singing along Millionnaire's Row, Great Orme this morning and although obviously a Willow Warbler it kept throwing Chiff-chaff bits in. It looked like and was a Willow warbler.
There is a recording here - http://soundcloud.com/marcbuzzard/mixed-singer-2
Try to ignore the noisy Blackcap. The chiff chaff bits can be heard at the end of the first and second Willow Warler phrase, while in the third Willow warbler phrase, it sings a couple of chiff-chaffs in the middle.
Here is a sonogram showing where it sings it. I've pointed the red arrow to the Chiff-chaff bits, and the blue lines generally show the Willow warbler phrase - the rest is Blackcap, Gulls, an easterly wind and me breathing noise. Is this is what is classed as a mixed singer?
Click on sonogram for larger image.