Agreed. I've seen several ball joints that have failed (and every single car i've seen with this failure has had it advised on the previous MOT which the owner clearly ignored), but i've never seen a chunk of metal like a wishbone rot through completely. OP, we'd need pictures of the failure point as i have a feeling that it has probably failed at the ball joint, and the wishbone hasn't rotted through. I've taken numerous suspension arms/wishbones off cars that are 20 years old and have 150k+ miles on the clock and none have ever been even in the same ball park as corroding so much that they rot through/fail. The only time I could potentially imagine this happening, is if a car had been parked in the ocean for 20 years.
And who is going to believe that the OP deviously removed his uncorroded and clean wishbone, found a wishbone online that was *almost* completely corroded through, got a garage to fit it, then put his wife and kids in the car and drove around as much as possible in the hope it would fail, just so he could have a chance at arguing with an MOT garage/VOSA about their culpability?
It does, but you have to look at it in the context of the alleged complaint - had this been a tyre that had worn away, and the OP had driven it for 7 weeks and 2000 miles since the MOT - then i completely agree. The tyre wear is possibly/likely to have been in a passable state when it was MOT'd, albeit you'd potentially expect their to be an advisory (if there wasn't some horrific alignment issue on the car)
But if we take the OP's assertion at face value - if a suspension arm has rotted through to the point of failure, this isn't going to have been a process that occurred in 7 weeks, or even 7 years. Hence the likelihood of it being in a passable state 7 weeks ago is very very low. And therefore the garage would have questions to answer
As I've alluded to above, the garage completely have liability if their trained technicians have signed off a vehicle as being safe and then a defect such as *significant* corrosion was found a few weeks later, which would have been present at the time of the MOT test because of the nature of the defect. Obviously this is with the caveat of taking *everything* that the OP has said at face value.