Andrew Gabriel wrote:

There's a table in the IEE On-site guide,

Some of the cable factors and conduit factors info is on the TLC web
site here
http://www.tlc-direct.co.uk/Book/4.5.3.htm.

You can also use the older rule that the net space factor - i.e. the
total cable cross-sectional area, including insulation, should not
exceed 45% of the net internal CSA of the conduit.

There will also be the issue of derating the current capacity
of the conductors due to grouping.

Indeed. The Cg factors are here
http://www.tlc-direct.co.uk/Book/4.3.5.htm
(use the "enclosed" column).

--
Andy