Zippy

Hi all First post on the site. Very happy owner of (Nov 06) 220D. Really enjoying car! Am looking at getting parking sensors fitted as my better half having trouble parking. Main dealer quoting in excess of £400 fitted. Does this sound about right? Have seen generic prices circa £200 fitted and colour coded (Cadoxton Slate). Would this affect compromise the Approved Used Warranty? Advice gratefully recieved