I dug out the speakers from the garage and assembled the sound system with the laptop, mixer and amplifier. I daren't take it past about 1/5th of it's capability but it sounded great. I have a strong urge to cover the speakers in fur. I think they'll look great with that and the grills I have on the way. There's still a problem with Traktor and the mixer on the laptop. It freezes up after a while. So more tests and troubleshooting to come.
I tried the 12v Battery to mains inverter today and though it ran the system comfortably there's a fault with the inverter and it kept going to fault mode. The fans didn't work either. I hope this doesn't turn into a long drawn out eBay drama and that they just send me a working one.
I'm going to buy a generator anyway but it would be nice to know the inverter works as I have plans for it .
(Oddly enough the inverter seems to have fixed itself now. I think there's a physical relay in there that was sticking but that's only a hunch. It's working now so hooray!)