One week later and its still not good. Pain seems more localised but mobility is still terrible. I'm going to do two week blocks of GTG push-ups and chin-ups because that's about all I can do, assessing after each two week block. I'm also walking in the evening when I can.
Before this, my weekly schedule with the 16kg bell had been one handed swings on Mon, Tue, Thu & Fri, two handed swings on Wed. Sat & Sun as rest days with more walking.
Full disclosure, on the third week I had done one handed swings on Mon & Tue with a 10 minute run after each because I was feeling so good
On Wednesday I did two handed swings and then Thursday morning I woke up with a problem. I'm still confident that its a muscular problem and nothing more serious. Logic says that after two weeks without problems, introducing the runs was what caused it. I'm still nervous about swings though because the problem is right where the swings would hit me - right at the bottom of my back and my butt.
So now I'm (over)thinking my options for when I start again (for a while, one option was cashing in on the price kettlebells are selling for on ebay!). I could just start again with the 16kg bell and avoid adding any runs. I could also ease into it with the program minimum from enter the kettlebell and then ramp it up with Swings and Get Ups 4 times a week and then back to five times a week.
I've also been looking at Geoff Neupert's Strength Shortcuts 2.0 but this is probably the kind of thing that's better to do after I've achieved Simple.
Thanks again for the encouragement everyone.