My unsupported span is 4.00 metres and old fashioned 7 x 2 inch joists
were used. This is a loft area and not walked on. To convert the area
into a living room the joists should be 9 x 2 inch or the new metric
equivalent.

Have a look here, there's a lot of info inthis site....

http://www.roofconsult.co.uk/

Chris.