please can you help me identify this bird

I saw this bird whilst walking yesterday perched in a willow tree in a small wooded area along side a  stream on farmland ( mainly orchard and grass) below the greensand ridge in Kent.  I cannot find anything like it in any of my bird books or online.  It was calling to another bird nearby with a sharp, repetitive call of a long chirrrup followed by two short twip twip calls.  It was really quite brightly coloured and not at all shy, which surely makes it unlikely to be a willow warbler or chiff chaff?  Any thoughts appreciated.