I use parallel jaw pliers, the kind with a screw to hold them shut, one tool fits all. The pliers come in from the side so the rest of the lock work never gets in the way.
The only trick is you mustn't put all the compression in the middle so put a metal bar along the whole bottom of the spring and clamp that. Tighten just enough to free the spring sideways so you can extract it before unhooking the stirrup.
If it's a frizzen spring, balance the frizzen half way back so the spring is as bent as it gets, bar underneath and just nip it with the pliers. Close the frizzen, remove the screw and out she comes.
I've had my share of broken springs but never from clamping them :thumbsup: