I have had to weld this immersion on as the thread was not in good condition prior to me getting there, and as far as I have been told the immersion cost 500 pound as is comercia style immersion so if that immersion has to come out it would be as well changing full cylinder
If you are an experienced plumber (and as a GSR you probably are) and can't see that its possible for the tank to have run dry it probably didn't. It'll be up to whoever claims that's what happened to explain how, on the balance of probabillities, that is what happened and that what you did to the system caused it to happen.
There are, IMO, other more likely explanations to be explored.
Firstly, it is always possible for a manufacturing defect to cause premature failure of a device like an immersion heater. It's unlikely (if it is a decent make) but you are asking here because something unusual has happened.
When you said you 'had to weld this immersion on as the thread was not in good condition' do you mean you had to weld in a new flange/boss to renew the thread or did weld the immersion heater directly? If the latter, did you do it in a way approved by the MI?
If the electrician made a mistake they may have wired the heater incorrectly, e.g. applying ~415V to some of the elements instead of ~240 volts. Was the electrician experienced and qualified to work on this sort of (commercial) installation? If it were me, I'd not want the wiring touched until it had been photographed and inspected by an independent electrician. There should be cutout(s) on the immersion heat that trip under fault conditions such as no water or incorrect wiring. You need a record of the state of those cutouts before someone resets them.
Be polite but careful what you say to the other parties in these circumstances. Don't say there was "No hot water." when you mean "There was flow from the hot water taps but it was at low temperature". Make sure 'evidence' is carefully collected and kept safely.