The Red-footed Falcon - Portland's sixth record - was a great performer © Martin Cade (video) and Pete Saunders (stills):
It's interesting to compare the extent of moult of this bird with that of our last showy Red-footed Falcon in May 2017. Today's individual had moulted precious little of its underwing coverts - we're thinking we can see a few 'new' grey feathers on the leading edge of the lesser underwing coverts on the right wing and on the greater underwing coverts on the left wing but could do with some more photos to confirm that. In contrast, the 2017 bird had big blocks of moulted adult-like grey feathers on both underwing coverts © Pete Saunders:
From an ID perspective, the lack of moult in this area opens up the possibility of Amur Falcon - they evidently often have an underwing pattern a lot like today's bird (and besides, they'd be acquiring new white feathers in this area that would perhaps contrast less well with the retained juvenile feathers than do the new grey feathers of a Red-foot). The literature seems to suggest that if you're lucky a first-summer male Amur Falcon will have a paler face, streaks on the breast and grey rather than blackish newly-moulted central tail feathers so hopefully you'd suspect you were onto something good before having to scrutinize the underwing coverts.