First off I would HIGHLY recommend you get a copy of the SiG Armorer's DVD. You can find it at such vendors as TopGunSupply. Invaluable to any SIG owner, really. Then DO NOT REMOVE the roll pin that holds the bottom of the mainspring assembly onto the frame while it is in it's pre-disassembly position (on the frame).

If you are impatient an wish to do the work yourself sooner rather than later, then at least watch one of Todd's videos on YouTube. He does a pretty good job with them and he works for Cusomized Creations, a refinishing outfit that also does customized gunsmithing. Here is one on the P6 (a Police issue variant of the P225): YouTube - AA Todd McKechnie Customized Creationz Sig Sauer P6 P 6 P225 225 Disassembly Video Part 1 3-09

Once you have the mainspring assembly off the frame, you can then tap out the roll pin, carefully, slide off the old spring and then replace the spring and put the roll pin back. Look carefully at the roll pin as they generally go in AND out from the same side and usually it's the left.

Shouldn't be any need for special tools, just a little time and some patience.

Just watched the video and I guess I'll be buying the DVD. I have popped the strut out so I'll have a go at it as soon as the spring arrives from Wolff.
Thanks for the info/links.
Cheers.

The mail guy returned with my Wolff shipment and I set to changing springs. The roll pin came out easily. It was tapered! Once I compared springs it was obvious that somebody had cut the spring to lighten the pull. After I put the new spring on and popped the roll pin back in I fired a mag of .40. No FTF and not a bad trigger. I also put 2 mags of .22LR downrange as I found the original hammer spring could not fire the rimfire round hardly at all. 32 rounds and not a problem.