It is clearly NOT the Collared Pratincole that graced Cley NWT Reserve (Norfolk) and then Swillington Ings (West Yorks) after its departure, as that bird had an obvious white trailing edge to the secondaries.
As this Sussex bird afforded excellent study opportunities, it was clear that it was in moult, with two of its outer primary feathers quite abraded. Perhaps as a result of this wear, there was no evidence of a trailing edge on either wing, and the tail feathers were particularly short. However, the bill base colour and extent, and the underpart pattern (marked contrast between buff and white) were typical of Collared Pratincole.
The fact that it had virtually no white visible led observers to naturally consider the possibility of Oriental Pratincole, a species so similar that it is easily overlooked. Fortunately, Jacob Everitt (amongst others) managed to take a large selection of photographs as it flew overhead and closeby and he very kindly emailed me a collection to study (see five of these published above). The state of the plumage is confirmed in Jacob's images, and in the odd image, you can just about see the odd whitish feather in the secondary line. It can therefore be assumed that the bird is a moulting COLLARED PRATINCOLE rather than an Oriental Pratincole.