Head took over heart
I was tempted to buy a 1,500 RPM back up gen but when I really did the use analysis I just don't need continuous running (at over twice the cost) so a 3,000 RPM unit will be fine. I was going to buy the Hyundai 5.2kw diesel unit (DHY6000SE) then I realised the Black and Decker one is a badged Hyundai and comes with an added 1 year warranty (2 yrs), free delivery, and £100 cheaper
It produces about 2.5kwh per litre of diesel so is reasonably efficient. It's far from silent at 70db but I don't have neighbours.