This is quite a saga so I will try to keep it as short and to-the-point as I can. Also, since I do not know exactly what the issue is with my car at the moment and if it is related to what's explained in this post, I will refrain from naming the dealership in question. However they are a massive company spanning many branches nationwide.

So I bought my used Peugeot 308 (2014) in Sept 2020, and I took out the 2 year service plan, the 2 year mechanical breakdown cover, and the shortfall insurance... the works!

I get a recall notice from Peugeot about the wet timing belt in Feb 2021 and I opt to put the car in for the complimentary check up. It's important to note I did not take it to a branch of the dealership I bought the car from as the closest Peugeot garage was another large dealership company, but they performed the checks and deemed the wet belt to be within spec. I have all the paperwork.

The car goes in for it's first service (of the purchased plan) in Sept 2021, around 2 months later the service light comes on and I call the dealership to ask for advice, I am told they might have forgotten to reset the counter and booked a date for me to come in to have it sorted. A day or so later the car then shows engine light and the STOP warning on the dash. I call the dealership breakdown line and they send out an AA technician to assist. He checks over the car and notes the errors are erratic but indicate an issue with engine oil, he then mentions that the car was recalled and i explain to him that i sent it in to be checked. He shines his torch inside the oil cap of the engine and informs me the belt is all cracked and has most likely shed particles into the oil and blocked the flow. When i told him the car had been inspected for this fault 6 month prior and it had only just been serviced he advised me that the wrong oil may have been put in the engine that has damaged the belt.

Car is taken in to dealership who determine that the wet belt is the issue, i complain and cite the documentation that says the belt was in good condition prior to the service and that their service is to blame, they agree to replace the engine and we fight over if I have to pay some of the cost since the cost of the repair exceeded the £2,500 covered in the mechanical warranty. After a few days i get a brief call to tell me the car is fixed and ready to be collected. No word on payments or anything so I take the car. You can tell it has a new engine as the car drives amazing after the repair.

The car was serviced back in October, now 3 months later the "service" light has come on... de ja vu.

I understand this new issue could be completely unrelated, but i don't want to take that risk, I want them to look at the car and I want them to check that the same mistake has not been made again... however my 2 years service and warranty are up.

I am going to phone their complaints line tomorrow about this issue as i don't want to just take it in for inspection then have to explain the history, i want this looked at carefully from the start so they are aware the moment it rolls onto their repair shop what the story is.

I suppose my question is what should I do? Assuming it is the same issue I had before, surely only their service can be the cause since it is supposed to be a brand new engine in the car... and if so they are liable to get it sorted warranty or not. Not to mention i should demand a full refund for the service plan i payed for the "peace of mind" when they may have broke my car on both occasions.

Also is there a way I can verify it is a new engine? They never gave me any paperwork about it, they said it is "internal documents only", but i do have the engine number on the documentation from the original check-up from Feb 2021, i don't know how to check the engine number in the car.

TL;DR
Dealership broke my car last year by servicing with the wrong oil and had to replace the engine, now 3 months after it's last service the car is showing same symptoms as before. What should I do when I call their complains line tomorrow?

Thanks for your time :)