The belt just went slack again. I thought maybe the tensioner bracket bolt repair had failed or something but I would have been surprised as a helicoiled aluminium bracket is much stronger than just a thread in parent metal.
On closer inspection it just seems as the tensioner spring had passed over top dead centre again but nothing broken. So I levered the tensioner bracket back towards its correct position and it popped back into position easily. Too easily if anything so I think the spring is getting tired.
I'll get a replacement spring.