Eddyman

This in in London Canary Wharf and no manufacturers marks to be seen. And I can't even see under the window frame because the window open towards outside .

So I have come across this handle as you can see in the picture that needs replacement. Which is fine I have replaced many of them in the couple of years but I have never come across one like this. The base plate can't be turned to assess the screws because it was installed against the window beading. So my question is did anyone has ever replaced a handle in the same situation and how did you do it? Thank you