So not having a torque wrench in my toolbox, I've over tightened the lockring on the cassette on a set of wheels that I own, and now I can't get it off – I've already broken one chain wrench trying.

Any suggestions?

get another chain wrench, a decent one.
Set up the chain tool and the spanner on the lockring at a 180 degree angle. Lean over the wheel, hand on each tool, psuh, HARD. Try and jerk rather than a steady push.

have never done this, but

? wrap an old chain right round the cassette (maybe twice) & then pull it tight into/with a vice to lock it

?? cut the lockring with a hacksaw/dremel

??? plusgas (keep it away from bearings & freehub though)

???? take to LBS who should have stronger tools

What Samrui says but still along metal pole on the end of the spanner too, I have a half length of scaffolding for doing just such jobs. There's nothing been able to hold that bad boy levering.

Thanks guys, sounds like a plan.

Bolt the lockring tool in place using the quick release, avoids slippage and maintains a good contact. If you've got a big secure vice put the tool in the vice with the wheel on top. Apply chain whip.

I use the chain-in-vice technique, works a treat every time.

Drac isnt a half length of scaffold pole three meters long. Theres over killl and theres over kill.