Hello Dave,
mid August in Cornwall and a strange looking bird. If its a rare bird, than I am out, but: why not a strange/ill loooking Willow Warbler?
Bill is too spikey and slender based for an Hippolais warbler, its also no Accrocephalus warbler with a notched, narrow tail and this bill.
Eyes seems closed due to an ill bird, hence the unusual look and I assume the unusual head pattern (moulting issue?). But its a long looking warbler with pale legs.
Happy to be corrected as allways, but I am in the Willow Warbler camp for this one.