Send a gunboat - Douglas Reeman

HMS Wagtail is a river gunboat, a ship seemingly at the end of her useful life, lying in a Hong Kong dockyard awaiting her last summons to the breakers' yard.
Commander Justin Rolfe is also seemingly at the end of his useful naval life, an embittered man, brooding and angry from a court-martial verdict.
Then the offshore island of Santu is threatened with invasion from the Chinese mainland. The small British community must be brought out and Commander Rolfe and the Wagtail are ordered to the island.
The job is regarded with sullen resentment by his crew, but to Rolfe, and even the ship, it is a job that offers the chance of a reprieve and a restoration of self respect.