Ahh, a Brummie. Tho I'd agree with the gradually increasing force.As a professional engineer I would suggest that this is one of those problems that can be overcome with the application of a hammer. Apply as many times as required, then bin it and deny all knowledge.
The thing was put together somehow, it'll come apart again. Looks like the right hand side would unscrew to me. Apply more force. Or a hammer. OK it isn't yours but as it is, unless the owner is putting you through some sort of intelligence test to see if you can open it (which BTW you've failed ) it's no use to them either.
Back to the hammer then.