If this has been an ongoing issue since before the warranty expired, you have a very good case to get the problem paid for by Peugeot, as it is clearly something that could and should have been dealt with during the warranty period.
I would approach the dealer in the first instance, and politely ask him to get in touch with Peugeot UK in your behalf regarding this repair, with a view to them covering the costs under the circumstances.
If you don't get any joy, I would get in touch with Peugeot directly, and explain what has been going on since you owned the car, and that this latest issue has been on-going since before the warranty ran out, and you have now been told about the rocker cover. (It's not your fault that the dealer took this long to figure the problem out)
If the polite approach fails, you should then try being a little tougher, and make sure that everything is put in writing, listing the problems you have encountered, when this fault was first reported etc, sending a copy to your dealer, and to Peugeot UK. You should call their customer service and tell them that you intend to seek legal advice if there is no assistance from them regarding the latest costs involved.
You could as a last resort go through the small claims court, as it sounds like you have a very good case if this issue was reported prior to the warranty running out, but it has taken the dealer this long to resolve the issue.
Hopefully, it won't come to that, and common sense will prevail when you explain the situation in the first place.
Good luck, and keep us informed how it's going.