Ok so it was
Fishy-Steve couldn't have summed it up better I think....
The fact they inserted a helicoil confirms that the bolt didn't fail it can only mean that the tensioner bolt was overtightened or wasn't tightened enough in order for the bolt to shear the thread or damage it on the way out.
Not sure how they can justify a bill of £1k for repairing this damage either - even if that happened under normal circumstances it shouldn't be anywhere near that. I think they may have just over inflated the cost and made you pay for the actual cost.
You are absolutely right that you should now take it to another garage and get them to write a report* and fix the fault (* get an independent engineers report too like RAC to back it up). Make sure that you advise the garage that you dispute the second charge work which failed and of your intentions to recoup everything from the small claims court. Make sure you summarise and approximate what this will be: bill for the damage #1 & #2, independent report #1 & #2, Transportation from garage #1 to #2, hire car, traveling costs, legal costs & any other miscellaneous costs you can think of plus perhaps compensation. I would give them one final opportunity to offer to fix the problem to your satisfaction and if you feel justified demand a refund on the obviously incompetent second fix.
Book an appointment to go and see the Citizens Advice Bureau they can give you free legal advice and help try to redeem the situation outside the courts.
If this doesn't work and it is a dealer group submit a small claims court intention to the Chief Executive* not the local manager/principle and get his attention or alternatively whoever the owner is (*I did this when I bought a car and was in dispute with the local dealer group - when I sent the court intention to the chief exec he just dropped his trousers and took the car back immediately).
I hope there is a silver lining to this cloud for you.