Big thank you to @Stephen!
When I finally managed to get on chat this morning the operator was rude and completely useless. The agent refused to check the offer, quoted £360pp as that is what the system says, and would not look into this Facebook promotion at all. Genuinely a terrible experience on chat.
I decided to call whilst waiting for responses from the chat agent, and the lady I spoke to immediately acknowledged the offer at £275pp and added to the booking!! The difference between the two channels was night and day.
Big saving on the drinks package for our first RC Cruise on Wonder next week (29 May) from Barcelona. Thanks again Stephen for sharing!!