Are you sure about it being a Short Sunderland? Imperial Airways and then (from 1939) BOAC operated a mail service from Poole to Lagos and Calcutta using Short C-Class “Empire” Flying Boats. These, according to my father who flew back to England from India in early 1939, were very luxurious.
In 1942 BOAC bought six Sunderland Mark III's which they modified as mail carriers. These were operated jointly with the RAF on the Poole - Lagos – Calcutta route. These aircraft had very primitive accommodation for seven passengers. I believe that there were no sleeping berths, only seats.
Later, I believe that it was in 1943, they BOAC operated a more sophisticated version of the Sunderland Mark II, the Short Hythe. This had better accommodation for up to 24 passengers, including 16 sleeping berths. BOAC operated 29 Short Hythe's.
For interior shots of the C-Class the following web pages are interesting:
http://www.seawings.co.uk/empwalkgal.htm
http://www.imperial-airways.com/Inte...de_saloon.html
http://www.imperial-airways.com/Inte...rd_saloon.html
http://www.seawings.co.uk/empiregal.htm General photographs of Empire boats
For Sunderland interior shots:
http://www.seawings.co.uk/SunderlandDUXFORD-WRgal.htm
http://www.seawings.co.uk/SunderlandHENDON-WRgal.htm
I do not know of any interior shots of the Sunderland Mark III conversions or of the Hythe's – but would be very interested to see some.