The machine would need to conform to the EN Machinery directive (CE Regs) and as such this covers design criteria for operators compartments, and so this would be the first point to double check.

If the machine conforms, which no doubt it does then I would suggest that if your companies insurance company is happy with the unit and your local HSE also then can you really continue to hold this stance?

If the operators guide, service info, a sticker on the machine or even better a CE certificate possibly sent with the machine detail that the machine conforms, then you have as much as the Manufacturer needs to provide to state the machine is safe in its design.