The pilot had been instructed by air traffic control (ATC) to make a straight in approach for landing on runway 03. The aircraft joined the circuit on long final as ATC cleared another aircraft for departure from that runway. While carrying out his pre-landing checks the pilot became concerned that the other aircraft's pilot was being too slow commencing her take-off run, and was still lined up at the runway threshold when the aircraft arrived on short final. ATC instructed that pilot not to delay. Just as the pilot was preparing to carry out a go-around the other aircraft commenced to take-off, so he continued his approach. During this time he had become so engrossed with the other aircraft, and planning alternate action, that he failed to complete his pre-landing checks and forgot to lower the landing gear. The aircraft landed wheels up on the runway.