Sorry if this has already been posted
I was looking on the BT website today about getting caller ID added to our package and it was £2.59 / month. However if you add the FREE BT Privacy at Home pack you get caller ID for free and they will also register you with the telephone preference service for no more pesky marketing calls!
You can add the pack easily online by logging into your account or by giving BT a call (I know which option I'd rather use!)
NB: You need to make at least 2 chargeable calls/ month otherwise you will be charged for the caller ID