Well, just to follow up, the installer has taken a look at the fire, and says that the full output is 4.2kW and cannot be reduced to less than 2.4kW without risking poor combustion. The problem this raises is that it has to be used in on/off bursts, and the only controls are near the floor behind a trim panel, which makes it awkward to have to keep on turning it on and off. In fact, so awkward that it's really impractical to keep on doing that. The controls are a manual gas valve and piezo igniter, therefore it can't be connected to a room thermostat.
The installer made an attempt to clean the staining of the glass, which was unsuccessful. He says it definitely should not have stained so quickly.
The owner says that when ordering she stipulated a fire which would be energy efficient and suitable for the room size in question, and I suggested she take this up with the supplier as in its present form it is neither energy efficient nor very suitable, therefore it is not fit for the customer's stated purpose. We'll see how this plays out but it seems to me that the supplier needs to rectify the issues which have been found with it. There is a similar model which can take a room thermostat, and that might be one option. I'll be able to get a look at it for a model number or photo early next week.