In The Case of the Ambitious Fishmonger, Mr Sinclair was an official with the Scottish Fisheries Board based at Elgin. He reported to the Air Ministry that some irregular night flying had been detected in the north of Scotland. When Biggles arrived at Elgin to follow up, Sinclair, a snady haired Scot of about forty, told BIggles that the aircraft was not a service machine or a passenger liner. It was a light aircraft which did not show any navigation lights and took a course into the North Sea always near the time of the full moon.
Mr Sinclair connected the flights with incidents of large-scale salmon poaching which had been taking place on the Scottish rivers he was in charge of. When Biggles appeared sceptical about the value of transporting salmon by air, Sinclair educated him with the numbers, showing how one salmon of ten pounds would fetch almost £7. An aircraft load might be worth some £2,000 - £3,000. Carrying so many salmon was like carrying so many five-pound notes.
When Biggles connected the poaching to an aircraft owned by Hugo Bikstein and found that he used a landing area in Scotland called the Culbin Sands, he reported these findings to Sinclair. Biggles suggested that they ambush and overpower the gang in the act of transfering the poached salmon into Bikstein's aircraft but for that they needed a strong force of men. Sinclair assured Biggles there would be no problem gathering the manpower--some of his bailiffs who had been beaten up by the poachers had been waiting a long time for just such a moment. Sinclair led the subsequent operation, involving his bailiffs, the Scottish police and some gamekeepers and ghillies supplied by Captain Mackenzie which resulted in the arrest of Hugo Bikstein's gang and the confiscation of the car and aircraft used in the poaching.