If the line really is DC and the mains is not supplying anything critical, it will be fine.

The important part of cable sharing a containment is that all cables are rated for the highest voltage present (230V) and in this case you do. Had it been bell wire there might have been more to say.

Actually some cheaper LED supplies are not that clean in terms of radio frequency interference, and it may be worth waving a medium wave portable radio near the cables to see if there is likely to be a noise problem - you do not want to couple high frequency hash from the LED driver into the building wiring, but in terms of safety you certainly can,

If in doubt aim for at least 50mm  spacing if you can, for example on opposite sides of the beam,

Mike.

Nathan Rose said:

I am wondering if running the 24v wire parallel to the 230v wire will have any consequences

Occasionally, there might be some EMC issues depending what the 230 V cable is supplying (how much current, inrush currents, and  how "noisy" from an EMC perspective). In general, though, over a run of 9 m, as Mike says, should be fine - try and include some spacing where possible.

Could try and put the cable in some flexiable conduit to give a better barrier but this will also alter its reference method and heat decipartion for the cable calc wich is a mute point as 24V on a 300v cable.  May be worth considering label the conduit along it length to highlight it is a DC cable inside. 

no need - a conduit will not help EMC unless it is metal and earthed at both ends. By all means label the cable if you wish to, but the regs do not require you to do that. Just clip it up - it will certainly be safe, and will almost certainly be fine EMC wise as well, just dont cable tie the DC cable to the mains one...

Mike

Thank you for all the suggestions. Where they pass through behind the rafter I have no space to separate them, but I can get about 30-40mm separation in the gap between the rafters. So I’m total they would be bunched for about 1m of the 9m run.