got a headlight out on dip - expensive?

[LX01UK]

Hi folks. I noticed tonight that my drivers side headlight is not working when on dip, but fine on main beam. Is this just likely to be a bulb that is somethign that is cheap to replace, or do you have to change the whole unit, which sounds expensive? How much should I expect to have to pay to get this fixed? or buy the parts and do it myslef. I have the workshop manual, but not had chance to read it yet, and this pc is too old to view it this week!

any help appreciated, thanks, Alex {never drives in the wet, or the dark anyway!}

Al.