In article <317758B5.10CA@edm.hull.ac.uk>, c.norgard@edm.hull.ac.uk says... [...]

>Can anyone explain why there is a difference in speed with different
>types of light?

Uh, the color response of film, and the meter, is not flat....
It is easy to forget that B&W often requires more exposure under
orange tungsten light than with daylight, or daylight-balanced flash. Same thing happens with color film when the metered subject is pure yellow, orange or red - more exposure than the meter indicates may be needed.
Hope This Helps