The principle is simple. Your car gets damaged not only when it gets dirty on the surface but when it gets dirty undereath. This is where salt gets into the wheel arches and elsewhere and you might as well start saving up for its replacement immediately.
So if you haven't got a garage with a platform onto which you can drive the car and clean it from underneath - which I'm guessing covers the vast majority of LifeOver35 readers - you need something that will turn the average jetwash into an underneath-the-car jobbie.
This is easy to assemble and does the job well. Typically from Karcher the water pressure is impressive. Yes, you'll need a good bit of space and an outside tap is a help - the distance between the kitchen in our terraced house and the pavement outside would make this impractical as a regular thing. Nonetheless if you have the set-up then the just-north-of-£80 including VAT I found on a website could be a very good investment that will extend the useful life of your motor.