Mary has had a poopy bum for a while and I wondered whether the smell was setting Peggy off. Anyway, I put Mary in the tub and then realised how long she'd have to soak before her poop clods softened, so I got the scissors and very gently and carefully snipped them off. She didn't mind. The whole process, from deciding to take action to cleaning out the tub afterwards, was over in about 30 minutes.
I can see Mary is pleased. The clods were rather large and would've been uncomfortable.
Peggy hasn't chased her since, but I'll keep watching.