I presume that 27 tonnes of GHG's for the one ton of uranium actually means uranium ore. If that is the case, how many tonnes of GHG's are needed for the amount of U235 (ie enriched uranium, ready for use in a power station) that represents the equivalent of 1 ton of coal?

I haven't found details supporting this number in the linked report. The reporting on production is in tonnes of uranium oxide (natural uranium rather than ore). So, you might want to work from there.