I would just buy another bottle and tell them to scrap it! not worth the mental anguish and at £100 thats more than half of the cost of a new one, plus I would be surprised if they used a Midland Jubilee valve which is designed for airgun use, not those horrible other valves which I see from time to time.

You could also buy your own Midland valve and just ask them to fit that, then you would only pay the test fee and that may really annoy them if all they are trying to do is sell you a valve that you dont need!

Truth is you will probably never know!