I've had new new top and bottom gearbox mounts fitted. Today the bolt through the top mount snapped for the second time. The engine dropped and driveshaft popped out again ?. Does anyone have any ideas why? Bravo tjet 150 2008

That shouldnt happen even od you mounted "low" quality brand. I fitted Lemforder.

The mounts are not the issue it's the bolt that go through the top mount, I had to get 1 from a scrap yard

Sorry, i missunderstand.
Cant buy new one with similar thread type and length?

If you're referring to the bolt that gos through the gearbox mount ... that bolt has no reason to snap.
I'd suspect:
-overtightening
-other mounts,... if those permit to much play then the stress would transfer in this mount....eventually to the bolt.
-other mount being badly aligned... could stress this mount
-play in driveshaft or in DMF ... will transfer much more stress to the mount each time take-off because the engine will get hit by the drivetrain, then the engine will hit the engine mount.