Couldn't be better
I'm very impressed with P&O Ferries and their payment of cashback. I've booked via Quidco several times - transactions always tracked and always paid. No problems at all.
We've also had great service on their ferries from Dover to Calais and return. The staff are lovely, and the ferries comfortable. We tend to use the Brasserie restaurant and Club Lounge though as they're more peaceful and we need to rest for a long journey.
We were delayed once and had to take a later crossing for which we were compensated.
Can't fault them!