An interesting amphibian, the Piaggio P.136 makes only a single appearance, in the derivative work Chappal Wadi, where it was being piloted by Lucellino Gaetano before being shot down by a Curtiss P-40 Warhawk. It's a pity, as the Piaggio had a good performance, with a range far exceeding the Short Sealand. It first flew in 1949 and would have made a better option than the Sea Otter for Biggles as part of his Air Police inventory. But remember, he had to "Buy British". The Piaggio also had a gull wing, just like the Gadfly. But production of the Piaggio stopped in 1961, so the type would have been old by the time of Biggles at World's End. Its engines were only about 350 hp rather than 1000 hp each, so the Piaggio could not have been the Gadfly.
The Piaggio P.136 as it appeared in Chappal Wadi. The registration I-FIMB does not appear to come from an actual, historical, Piaggio.