£3500 is a disgrace !!!! - The tensioner doesn't actually have to fully fail to cause this problem. Its the chain which is the problem. If the chain over time gets too slack it will come off, the tensioner should pull it tight, if for some reason the the ratchet doesn't spring back / pull tight the chain will jump.
So keep fighting and push really hard, There is nothing else that can cause it.
Is it still under warranty or less than 60,000 miles ( engine and transmission warranty).