Loose and cracked bricks in loft?

If the mortar bond between the blockwork has broken to the extent that you can move the wall then I would recommend you have someone look at it. I would get somebody independent from your builder.
As a temporary measure, I would use some timber strapping, fix it securely to an area of wall which is sound and then fix individual loose blocks to the strap to try to tie it together.
I hope you get it sorted out.

What's going on under all that mortar buttered over the blocks, was that the builder fixing cracks after jacking up? Might be worth measuring the height between blocks to work out how deep that mortar bed is underneath... And a straight edge across to see how level each course is, to see if it's puckered up.

I have been told a crack is nothing to worry about unless you can get your hand into it.  Buildings move around especially after major work.  I would get it checked but I wouldn't loose any sleep over it

I wouldn't be so worried about the cracks if it wasn't for the fact that

you can actually move the wall just by touching it in certain areas.

I'd suggest following Sandboys advice and putting in some timber strapping as a temporary measure.

OOft - missed that.  sorry

Yeah, it’s one thing having cracks in a finish surface like plaster - completely normal - but cracked blocks in a gable wall are another matter, and if the wall is unstable (ie it moves) then something really needs doing. I’d be getting an expert in to inspect. I am not an expert, and I don’t even know who I’d phone first.

What I would do in your scenario is post to the screwfix community forum, I’d be hoping to hear from one user in particular - abrickie- who often gives good advice. Not many brickies or builders here in singletrack but there’s a few that still might comment here.

Those interior blocks aren't load bearing but at the same time won't be held in place by the weight of bricks above. If the interior are loose they aren't bracing the exterior face. Gable ends are the walls that tend to collapse in strong wind. I'd say they need looking at and rebuilding. It's possible during the extension work if they did Jack the wall up too much they've popped the top courses. Either way I'd wantvti know what caused it.

Please don’t worry about the gable wall to much. They are Selcon blocks which are a PITA as they suck moisture out of the mortar when they are laid. They are better under load.
Selcons are not ideal in a loft as loft temperatures change rapidly so will cause cracking along mortar joints.
Your better off trapping the wall in between timbers fixed to the underside of the rafters. Sorry I don’t know how to add a sketch to the post. Happy to give more info if you want to contact me. Email in profile , I think 🤔
Hope that helps.
Ed.

Taken into to loft because they are lighter than a feather?
We used to dunk them in water or the edges at least to stop them sucking up the mortar mix.
Fixing to them is a pain as well.

When edthecarpenter talks about trapping the wall between timbers - you can see a timber either side of my gable that do that in this pic taken before the concrete was poured:

We've in a seismic zone so the gable has reinforced concrete up the middle and over the top. The roof timbers are screwed/bolted together and onto the walls. You can also add cross members to triangulate the rafters - I haven't done this as the gable is only 1.7m above the belt of reinforced concrete around the top of the wall.