Cracks mortar in outside bricks under window.

[8misskitty8]

House built in late 80's and windows were replaced with upvc Windows approx 15yrs ago. We bought the house about 5yrs ago. We noticed a bit of mortar was cracked in a few bricks under a bedroom window when we moved in but thought nothing off it. There are no corresponding cracks inside.

Recently I noticed the edge of the cil outside is slightly lower than the rest, right above this crack. Not sure if it was like that when we moved in. Me being me I leaned out the window to get a closer look/to have a feel and a lump of the concrete mortar came out on my hand.

What can I do about this ? Do I need to replace the window and bricks or Is it something worse like subsidence ?