Long Island is the largest of the islands off Rio; it is based on Belle Isle the only inhabited island on Windermere. Hence it is probably the One big one (which) had houses on it (SA8)
Long Island in The Lake appears in Swallows and Amazons, Swallowdale, Winter Holiday and The Picts and the Martyrs.
... the long island that lies in front of the town ... (SA8).
The steamer was coming out of the Rio channel between Long Island and the mainland (SD1). Swallow and her convoy were moving in between Long Island and the shore towards the little town of Rio (SD8).
When the lake freezes over the ice round the steamer pier and out beyond Long Island was black with skaters (WH9).
The little boats had a fair wind now and were running before it between Long Island and the islets to the west of it (PM15).