AFAIK, all UK Chipmunks civ and mil had the strakes fitted as a Mod, and the Mod also increased the chord length of the rudder.
The aircraft is cleared for up to 8 turns of an erect spin. As the spin develops, it becomes flatter, and recovery eventually very difficult.
There was a lot of folklore about Chipmunk spins. Do it by the book and at a safe recovery height and be prepared for the recovery delay to increase with the number of turns completed.
Anyway, who would want to spend all day getting to sufficient altitude to begin an 8 turner? 250 ft per turn and 1500 ft for recovery and, as per then RAF rules, recovered by TL plus height of ground.....