Bazza,
From Dave's pictures, it looks as if the spring may be the same as those on postwar brake cables. They only have a light action and are intended more as a telescopic dust cover. Secateur springs have a much heavier action and, usually, much shorter movement - resulting in a much heavier feel to the clutch. If you're a member, LDMCC postwar spares can supply a new brake spring from stock.
Regards,
Eddie.