Thanks for the comments above.
I was also concerned about that much current. The cable is not that chunky, but the sticker on the back clearly states 180W. It works great when connected to the car, although it’s not as powerful as the water pressure was when I had an outside tap in our old place.
I bought Maplin’s £20 5A adaptor and the light flashes and the washer just chugs once a second so it’s clearly starved of power. As with poster above the Maplin adaptor is back in box to go back!
I might just get an outside tap fitted here, but as we are now in a rented place it’s a pain rather than a £15 trip to B&Q.
Washer is this one: http://www.amazon.co.uk/Laser-4289-Volt-Pressure-Washer/dp/B001QFJRDY
It was quite a bargain (£35) and as per line above perfect, other than not being able to run it on mains.