Thanks Vinrouge & Nutloose.
I hadn't thought the cost would be lower - was resuming that RAF transport aircraft are effectively 'sunk costs' so only fuel and crew costs to cover. Different economic thinking.
Nutloose - I like that idea, though I thought the Yangtse river incident ships were heading only for Nanking, which is downstream of Wuhan, isn't it? And they had a hard enough job getting out as it was...