Asking for someone in an FB group.

Owner bought pup in November, was promised papers soon after and has not received them.

Serbian father, which apparently has "complicated" the KC reg process for the litter.

Is that true, and how long is it taking currently in general?

https://www.thekennelclub.org.uk/shop/registrations/form-1-litter-registration/
Click on the tab for FAQ, that will then show this:
We have used an overseas sire. Can we register the litter?

You will need to provide us with a certified copy of the Three Generation Pedigree issued by the overseas kennel club. This will need to confirm the pedigree details and the current registered ownership. We require this to be submitted with the litter application. There is a fee to record the sire. If the dog in question has already been recorded, we require no fee or pedigree to be sent, unless, the ownership has changed.

I don't think its that complicated - I have heard about this problem recently, mention trading standards if they were advertised as pedigree/KC registered