My advice would be to not buy a home on this site.
I could guess as to the cause, which would be the high winds (Storm Eunice) a few weeks ago caused the grey coping to flap around and crash on the last course of bricks causing them to be dislodged. Weak mortar or not, this would happen again and again.
In addition, the joints of this capping Plastic or metal is unsure) are badly finished or not jointed at all.
And it all looks way out of level too.
Looks like a David Wilson site (aka Barratt)