The pilot stated that he had experienced a landing gear retraction problem the previous week which was checked by an LAME but no faults found. On the day of the occurrence he decided to fly a circuit to verify the landing gear operation. After takeoff the landing gear retracted normally. The pilot continued with the circuit, maintaining separation with an ultralight aircraft also operating in the circuit area. On the downwind leg the pilot reported the aircraft suffered a radio problem which he attempted to rectify while still maintaining a lookout for the ultralight aircraft. This allowed his attention to be diverted from operating the aircraft and failure to carry out the pre-landing checks, including landing gear extension, resulting in a wheels up landing.