As you say, you are assuming ... and they should have ...

Engineers report is what you need. Without it, you have nothing.

How do you suggest I obtain an engineers report of the work carried out?

And I know this stud was removed and re-fitted (not replaced with new part) as there mechanic told me.

As I said, you would have to prove - to a high standard of proof - that their work or workmanship fell below a 'reasonable standard' or was negligent. That is usually done via an engineers report - I'd suggest you get in touch with the institute of mechanical engineers, and they would be able to advise further, depending on your area.

Bear in mind that the very first defence of the garage would be that they supplied and fitted all the items that the kit came with. Whilst it might be 'common knowledge' on internet forums that the stud can fail, if the manufacturer does not insist on (or recommend) replacement when changing the tensioner, then the garage is almost certainly NOT liable. A 'normal' garage cannot be expected to know every single detail and foible about every single engine/gearbox/etc. on the marketplace. (though if you'd taken it to a VW dealer for the work to be done, then you could argue that they should have this knowledge)

In addition to which, the simple fact that it took a further 7 months for the bolt to fail adds to the argument that this was a simple failure, which could not be predicted, and that the bolt was in good condition when re-fitted.

As I said in my first reply, I think the garage is not liable - for the reasons listed above.. I appreciate that this is not what you WANT to hear. But I also think that you'd have a hell of a job showing that the garage were negligent, or that their workmanshp fell below a 'reasonable standard'.

Another thing you have to factor in is the age of the car. You don't state how old it is, but if it's a Mk V it has to be at least 7 years old, and with over 100,000 miles on the clock and assuming it's always been privately owned suggests it's older than that. Unfortunately this means it isn't a new car, and therefore will have been subject to all sorts of wear and degradation during its life. This will make it even more difficult for any repairer to predict future failures, and would weaken your case against them even further.