Unidentified large bird

Hi, I’m hoping someone could help identify a bird I saw this morning at Rye Harbour Nature Reserve today (29-9-22).

it was a large bird, black in colour with brown blocks toward the tips of the underside of both wings. I think there may have been white either on the tail or on the head, it was a little distance from me. The bird was gliding over water and grassland and the wingspan looked pretty large, with rounded tips on the wings. It then flew higher and glided over the nature reserve and wetlands, heading toward Dungerness. 

any help or suggestions as to what this bird could have been will be greatly appreciated.