I will try to provided some insight into your problem based on my experience. However, I am not familiar with British design codes so my answer relates to what is common in the U.S.
In a system where you have a number of parallel rafters, which are loaded by decking or sheathing there will be some lateral distibution of a concentrated load on one rafter to the rafters on each side.
You could refer to ANSI/AF&PA NDS-2001 section 3.2.2 and section 15.1 for information on this topic. Section 15.1 gives some factors which can be used to estimate this distibution.
Also if you are dealing with closely spaced rafters, 16" o.c. (40.64 cm ?), the NDS allows you to apply a repetive member factor, which partly accounts for load distibution. The repetive member factor allows you to increase the allowable bending stress by 15%.
You can also refer to the NDS Commentary for more information. The NDS Commentary references a more complicated method. The reference is: National Forest Products Association "Alternate design procedure for wood floor joist system". You could contact the American Forest & Paper Association,formerly NFPA, for more information.