Hi Fenners,
Here is a link to the TRADA span tables that I recently used:
http://www.e-lindsey.gov.uk/NR/rdonlyres/85BC86BA-F27B-43CA-8080-F878C5CAC9AC/0/TradaTables.pdf
Be wary of the difference between C16 and C24 grade timber as there are separate tables for each.
HTH,
C