I am trying to get my cut marks to appear in Black only, at the moment they are appear in all 4 colours, even though I have the Gray Value set at 100%. Is it possible and if so, how?
On 126.96.36.199 or later, the crop marks added by imposition node should be black only i.e. using K channel.
If you get different results, please escalate this to hotline and they can investigate further.
If the crop marks are in the file and is defined in 4-color, then you could use the Optimize component to delete the original crop marks. After that you could add the crop marks with the Impose component which will be in black only.
Another option is to convert the existing crop marks to black only with a Preflight Fix. See this post for an example of such a Fix: Cut Marks in CMYK or Black "ALL"