Cracking on extension brickwork, subsidence?

[Webbers]

Hi,

We bought the house a few years ago. It was built in the early 70s and a 2 story extension was added in the 80s. The gable end of the extension is joined to nextdoors garage (shared wall) and the upper gable end is built above. I have only recently noticed this crack when I had to get on the garage roof to alter the ariel but it could have been there for years.

The crack starts 2 bricks above the garage roof and is about 5ft long. It seems strange that the building would move this way. Sun is on that wall
Most of the day.

I was going to get peoples thoughts on what might have caused the cracks and how best to fix it? I was thinking about grinding away the cracked mortar and just repointing?

 

[Webbers]

The red arrow shows where the cracks are on the gable end.

 

[^woody^]

Whats below?

Cracked on the inside too?

 

[Webbers]

There is a kitchen on both sides below and no cracks below. I think the wall below is double skin breeze block