Can someone tell me what this is please

Just a guess, but isn't it the Operating Handbook, QRH etc.?

 

I mean, that would make the most sense.

I think it also relates to legal papers such as insurance certificates as well.

I had not heard that term applied to aviation before, but in this context Tony is close.  In the US, regulations require all airworthy aircraft to have certain documents in the aircraft including current weight & balance, operations manual, registration, etc.  If you pan around the cockpit you'll notice some legal looking paperwork posted on the aft bulkhead.

Thanks guys