Only time I've had a strong smell of avgas was just after take-off from the strip, so I came back PDQ.
Turned out I'd tucked the fuel strainer into its rightful place in pax footwell pocket, but I hadn't emptied it completely: there was about 4cm of avgas in the bottom of strainer.
No idea how it happened but you can bet your sweet bippy it hasn't happened again.
Safe flying.
Cusco