OUR help is being enlisted by John Dunstan, who served on the HMS Black Swan, which in April 1949 went to the aid of the HMS Amethyst when it suffered severe damage from enemy fire while en route from Shanghai to Nanking. The unfortunate event came to be known as the Yangtze Incident. Writes Mr Dunstan, who lives in Devon, England: ''Most of us who were involved in the incident are now in our seventies and we are planning to return to the Far East while we are still able.'' About 200 of them are planning a trip to Hongkong and China in May or June next year, 45 years on from the incident, and would be grateful for ''any assistance, financial or otherwise, that your readers might be able to offer to make our visit possible''. So how about it, folks? All offers of help to Keeping Posted, please. We'll pass them on.