To check the internal mortar, a section of the block wall would need to be exposed and some mortar samples taken inside your house.
It is normal practice to build the brickwork first so it could be that the blockwork may have been built with a different mix, especially is coloured mortar was used for the brickwork, or if a new delivery was made, which is probable at sometime in the superstructure build of your home.
The NHBC are not being fair in my opinion, perhaps you should leaflet EVERY house on your development and tell them of the weak mortar, or tell the NHBC that you will?
So much for the "peace of mind" warranty!