I will tell you what I know and what I am looking for. The date was either in 1916 or 1917, and my sources are from Royal Flying Corps records and pilots diaries. Some of the dates conflict so I will confirm them when I have checked Royal Navy records.
Here is the story: the French and British shared an airfield on the northwestern coast of Thasos, east of Salonika and opposite the port of Kavalla. The purpose of this airbase was (a) to provide reconnaissance and defence for the Allied troops in Salonika and (b) to provide anti-submarine patrols off the Macedonian coast and in the waters between Thasos and Mudros. It was widely believed that U-boats were operating from secret bases along this stretch of coast and Kavalla was a favourite suspect. Reports came in that the Kavalla Customs shed and it’s enclosed dock were giving shelter to at least one German U-boat, through a “private arrangement” with the local authorities interested in protecting their fishing fleet. The Royal Navy sent a 14” gun monitor, name to be confirmed, from Mudros to bombard the Customs shed with the RFC from Thasos providing artillery spotting. The bombardment was successful in that the Customs shed and it’s dock were pulverised without any significant damage to the harbour or other shipping, but the RFC lost a plane to friendly fire when it was hit by a 14” shell while spotting!
All the above seems to be fact, and I am working on filling in the details of dates, ships and personnel. There are several contradictions in the records I have checked but there is no question that the events described actually happened. What is very much in doubt is whether or not any U-boats were sheltered in the Kavalla Customs building. I will dig through the Naval records to find any sources of the reports that lead to the shelling, but there are none quoted in the RFC records.
I will produce a book on the episode once the facts are established. I have visited Kavalla and found the long forgotten airfield at Thasos (now spelled Thassos) and I am convinced that the atmosphere in eastern Greece at the time of these events could have allowed a local authority to look the other way in matters of breaches of neutrality, as the country could have gone either way. There are many unsubstantiated stories of “secret bases” being used by U-boats: I wonder whether this one will turn out to be true.

If you can shed any light on the subject of official or “secret bases” I would be grateful.

Regards,

Bruce Dennis