If you mean the "guy" removed the thermostat then get it replaced. Thermostats are an essential part of the cooling system, not only do the control temp but they also regulate flow to the correct parts of the engine. manufacturers fit them for very ggod reasons and running without them is never a good idea.

If it still looses water after replacing it (if it is actually out) you need to get a pressure test carried out.

any other ideas?

Read my previous post. Put the stat back in them ask for more advice if its still aproblem.

The thermostat is in but just stuck open, ensuring water is circulating

But the thermostat is not operating as Vauxhall intended and they know a thing or 2 about these engines.

Replace it with a working one and try again.

Don't get me wrong I am no expert on cars......but surely the water is going somewhere whether the thermostat is working or not?

Of course it does but not necessarily to thr correct place at the correct flow rate.

Fit a stat and then see how it works.

As I said above, Vauxhall know a thing or 2 about engiones and fit stats for a reason.