A boat came from clear astern straight into the gap below me — did I owe her room?

Yes. Under Rule 19.2(b), when boats are overlapped at an obstruction, the outside boat must give the inside boat room to pass — provided she has been able to give that room since the moment the overlap began. A boat you must keep clear of counts as an obstruction. The leeward boat hoped Rule 19.2(c) (clear-astern boat not entitled to room at a continuing obstruction) would save her, but a boat under way is not a continuing obstruction — so 19.2(c) doesn't apply. The only test left: could you have given room from the start? If yes, you must. (World Sailing Case 29.)

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<p><a href="https://sailing.kiwi.nz/run/case-29-continuing-obstruction-test/">Source: Sailing Kiwi</a></p>