Thank you all kindly for all your helpful answers!
Had to knuckle down to a spot of sustained work, so haven't made a lot of progress - but have modified some of my work habits so everything can be done on the laptop if necessary - and I reckon (with the kit I've got so far) I've got about twelve - fourteen hours endurance on the laptop.
Still trying hard to avoid going the generator route - may take a look at solar when I've got a little more time.
Thanks again everyone - will post again when I've gained a bit more experience.