One step on the road to recovery!
When I let the tablet die in download mode (rather than when seeing the battery icon, or the samsung logo) then it did indeed start charging properly and tried booting up (got stuck on the "pulsing" samsung logo - but definitely a step closer to revival). I'm flashing a stock ICS firmware now.
I'll report how it goes, and will try rooting and loading other firmwares if successful.
I think it's back.
Everything is up and running so certainly happy about that.
Thanks for the excellent from daniel_hk and coco'z. If anything else hits the skids I'll be sure to ask for help here :fingers-crossed: