the other side of the wall is external bricks via a cavity as its a garage attached to a new build house. I was thinking maybe use the mortar course for the top fixing and then re drill the pull up bar bracket to match the next mortar course down, are mortar courses with Thermolite block strong?Maybe, but I would suggest you would need to distribute the load - Bolts through the wall, to the far side, then large washers or some steel to spread the load.
are mortar courses with Thermolite block strong?
That depends entirely on the strength when it was mixed. Fixing to the joint, isn't usually a good option where there is a heavy load expected, like this, but the morter might be stronger.
The alternative would be to find some method to get something in the cavity, at the back of the Thermolite blocks, to distribute the load. I'm thinking a strip of steel, ready drilled and using threaded rod through the blocks. Have you any access from above the cavity, to lower a bit of 5mm steel strip into place, then fit the nuts?
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