Don't bother. Nak have withdrawn from the UK, because of this you can get their latest h/u quite cheaply as this is surplus stock companies are holding. The reason they have withdrawn is because of the quality of their equipment is below par and the majority of their units have broken or have random unrepairable faults.
If you want a high end SQ unit look at Alpine (pre glide touch rubbish), Becker, Blaupunkt and Clarion.
Mike