Timing belt tensioner failure

[monty400k]

Started my car yesterday after it had been parked for a few hours on a steep slope, straight away it sounded terrible so it was quickly switched off, its a high mileage engine but had belts, tensioner and pump replaced 3 months ago so I was at a loss as to what could have happened, thought maybe low oil and the angle it was sitting had starved it but when I rolled it onto the level the oil was sitting perfectly at the max mark on the dip stick. Towed it to the garage that did the work, apparently the tensioner has failed, the cams are in time but it is 3 teeth out at the crank.

The garage suggested as it was parked on a slope the hydraulic tensioner wouldn't have had enough pressure but this sounds like nonsense to me, they are going to retime it tomorrow however I can't say I'm confident that the valves will have escaped damage, it still runs but sounds terrible...