2ManyTrucks - thanks that's great information; I searched but clearly didn't search enough. Just from what you say there, sounds like the issue is likely they need re-magnetizing? I'll have another search here and read up more on this.
Well, neither worked before I tore into 'em so if this doesn't work then I'm no farther behind

. If it quits pouring rain I'll try it out. I have a new unit on the way regardless but it takes weeks to get anything up here.
I do have the broken piece, I'll try and epoxy it back together. Nice to have some spares, if they can be salvaged.
Edit: Rain stopped, I test drove it - and it's no change, even with the swapped drum. 2ManyTrucks I found one of your other posts now on how to re-magnetize, wish I'd seen that earlier before I got carried away. I'm sure I've tweaked both those springs now. Another lesson learned the hard way