I removed the ridge on the drum (the wear ridge makes removal awkward) thus.
Took the drum off...refitted it back to front with some washers behind the nuts to allow fitting.
With that wheel off the floor engaged drive...the drum spins.....and used a piece of cobalt (8mm sq section?) that I bought for a few pounds from an engineering supply toolshop.
Not as tidy a job as an engineering shop would do....but only took a few minutes and the cobalt cut the steel ridge well.
Don't know it this is a good tip or not...but I was impressed!