id check the colour space you are outputting to for the pdf and either ensure they are the same or specifically pick a profile to output to.
also might be worth checking the pdf results by opening in photoshop just to see if they are at least digitally the same before they go to actual ink.
and lastly if they are different when checked in photoshop,
have you tried placing the .ai file into an indesign doc, and then saving that to pdf (and then check in photoshop) so that they both go through the same in-design output to see if that changes anything?
I have just exported both and as expected they are the same when the PDF's are opened in Photoshop - but as I didn't export them when the issue occurred (not my job originally) I have no way of knowing if the original designer exported to the same colour space etc so I can raise that too. Thank you for your advice.
i'd say thats the problem then, as until you export it its all kind of un-determined.
you can see the difference easily yourself just by saving a pdf one with Fogra39 under the colour destination settings on pdf output page/ and one with generic CMYK for example.
the other thing is that even printing the same job with the same printers (without re-supplying artwork) i've had different results before, i know loads of print aficionados will say its all BS and that your printers are rubbish (he may well be - i'm not working with ground breaking budgets here) but it seems that there are variations in the system just due to the ink and machine set up, or which machine they are using this time - which can change depending on the volume you are ordering.
also, one last "get you off the hook bit"
if they have changed designer, they have possibly changed printers too since the last job, to try and save money?
i'm sure there is no way you'll get exact results from one print business to another.
splashing out for spot colours is the easiest way to get close each time but then you're into more print costs then,
which inevitably is not the client goal when your fixing up previous work done elsewhere.