This is the end. Got round to buying a new third party adapter (65w), did the logical thing and turned the Mac off, bearing in mind it has a new third party battery in it with some charge left in it from when I bought it, plugged the new adapter in and... It won't turn on.
Even with a PMU reset (power button down for five seconds or something), no difference - just cleared the date and time as it does. I thought this was taking this piss so I turned the Mac on with the old adapter (new battery still plugged in too), when it had powered up I quickly pulled the old adapter out and plugged the new one in and... The PowerBook recognised it and stayed on. Ah, so it was taking the piss.
If however I turn it off and try and turn it on again with the new adapter, yet again it doesn't turn on. No light, doesn't even chime. Bloody thing. So I think that's pretty much the end of it. This PowerBook's power management unit is kaput. And seeing as I still have no clue if it's on the logic board or part of the backup battery (and I'm not spending any more money on the ****ing thing to find out), I truly have now given up on it.
Eventually I expect the PMU will just die completely and I won't be able to turn the thing on regardless of what adapter or battery I use. So I think I'll consign this to the junk heap. I've got a G4 MDD, I'll just trandfer what I need to that and this laptop... Well it served me well but it's completely useless to me if I can't move it around at all without having the adapter plugged in. I might just as well use the MDD.