Thank you for taking the time to reply.
I have indeed found the colour settings tab, and I've got a colour profile downloaded from a previous similar project (back when I had access to adobe).
To be honest, I've changed the settings around a fair bit trying to figure out the multitude of options and pdf versions, so I'm rather confused now. But starting from scratch - select pdf 1.3 (deemed to be the 'safe' bet?) in the preflight verifier and in the pdf export compatibility selection, select colour output intended for as printer, and then tick the activate colour management box, changing the cmyk sections to the icc profile that I believe the printing company uses.
I've now realised that the pdf/x output intent thing is only applicable for when I select pdf/x in the preflight etc, (duh!), so now I'm wondering whether it's better to have a pdf/x-1a file that might not be compatible with the printers but definitely only has cmyk colours, or to use pdf 1.3 and potentially mess up the settings and end up with rgb in the file.
I hope that makes sense, though my apologies if not.