SFB Controller described a near collision event between two aircraft in the VFR pattern during a busier than normal traffic period; when one aircraft elected to make an unexpected 360 degree turn.

Narrative

While working the Local Two position with multiple aircraft in the pattern; A SR22 called inbound from the southwest for pattern work. The aircraft was given a turn towards the airport and a descent to expedite his entrance into the busier-than-usual traffic pattern. The SR22 requested clarification of the previous clearance which was given in addition to traffic he was to follow on midfield downwind; another Cirrus. The SR22 reported that traffic in sight; an option clearance was given to follow the leading traffic for Runway 27L which was also read back correctly. There was a transmission heard/recorded that sounded like there were two aircraft talking at once and I replied 'blocked' to which there was no reply or retransmission. One mile south of the airport the SR22 made a request for a left 360 because they lost sight of the traffic to follow now on short final Runway 27L; there was an aircraft behind him on the crosswind and I had told the SR22 'negative' but he was already turned and flying westbound at that point right into the path of a C152 behind him just turning downwind in his face eastbound. Both aircraft were given an immediate turn to the south to avoid a collision and a re-entry for the left downwind. Perhaps a TCAS system requirement for smaller aircraft could have prevented this incident.