I didn't have the PCB changed - I posted the instrument cluster to Cluster Repairs UK, who diagnosed a fault with the connector and burn marks on the PCB. They fixed the connector and cleaned the PCB, before re-testing to confirm it worked + sent it back to me.
In my case, this resolved the issue.
Your first step is to follow the YouTube video in the first post on this thread, so you can remove the instrument cluster. Then if you move the cable connecting into the back of the instrument cluster, see if you can get the warning lights to come on and off.
If so, I'd say it's 80% going to be the instrument cluster. You can either follow the instructions on this thread to repair it yourself, or send off to a company for repair. I used the company mentioned here, who charged £160 inc. return postage, which resolved the issue and means I now have a 2 year warranty on the whole cluster (yes, not just on the repairs they made).
If not, it might still be worth having the cluster sent off, as if they can't find a fault, it'll only be around £40 for the diagnostic.
As others have said here, this problem is becoming more common on the MK2 Focus. Dealers are also reporting this, based on my research.
Last thing you want to do is spend £800 parts + labour for a new instrument cluster - all keys need to be recoded, and the instrument cluster has to be programmed to the vehicle + immobiliser. It's a pain!
Any problems / questions, let me know. It's thanks to this forum I saved a ton of money, so I'm determined to help others do the same.