Thank you for replying. What is the best way to rectify this, as there is no mortar holding the bricks together. I put a regular knife in to one of the cracks and it goes all the way in, to the depth of a brick. Also water can get in, which is a concern.
Please see attched photos of one side of the extension. The other side is exactly the same.
The extension was built 3 years ago and has been fine up until now.
I would appreciate any thoughts on potential cause, and what are the next steps.