Have just found out that apparently an 800kg aircraft parked on the ground weighs 800kg and has a mass of 800kg.
Mr Newton, indeed anybody with GCSE Physics, would strongly disagree.
Mr Newton wouldn't disagree. Historically, grammes, kilogrammes and pounds have invariably been used for units of force, on the basis that the conversion rate between weight and mass is, for all practical purposes, standard. The newton as a unit of force is a relatively recent invention. Principia also refers to "centre of gravity" (gravitatis centrum), by the way, so it's hardly surprising that the term stuck.
I haven't read the text of Pratt's book, but if the most serious crime he commits is saying that an aircraft "weighs 800 kg" then it doesn't really dent the physics.