moisture. i cannot tell from the pictures what these bricks are but just keep an eye on it from time to time. in the very worst situation an odd brick can be chopped out and replaced if it is considered that it has failed. i think you are a long way from that.
there are houses that have been up hundreds of years with very shallow foundations so i think that should put your mind at rest.
thank you very much for replying so quickly, really appreciated. 