Shortly after a normal touchdown directional control problems were experienced and the left wing began to lower. The pilot applied full power and carried out a go around. The pilot advised the tower that the left mainwheel tyre was probably flat, and subsequently positioned the aircraft for another landing. Normal gear down and locked lights were illuminated, however after touchdown the left gear leg began to collapse and the aircraft slewed off the side of the runway. The left gear retracted due to insufficient overcentre action on the side-brace lock. This was compounded by wear in the side-brace centre-pivot. Both of these potential problem areas had been addressed by the aircraft manufacturer in relevant Service Instructions. Compliance with these instructions was not mandatory and the aircraft had not been modified.