Thank you for your replies.
A week further on and I am no nearer having this resolved.
I went back to my TA and advised that it should be covered by the best price guarantee etc. After four days (by which time the prices/offers had changed) and said that they had said that I could upgrade from our carefully chosen 4D cabin to a 2C cabin on deck 7, which I refused as deck 7 is where we want to be. They advised that they would have to:
"keep calling them in the hope that there is a cabin available in your category as if we are booking again, so that RCCL can see the current price and offer the PRICE MATCH".
I then pointed out that there were several 4D cabins available (as RCCL was in the middle fo handing out Royal Up upgrades) and the response was:
"They have re calculated that cabin and as they see this as downgrade the price will work out more at £2732.48. When checking on the system, RCCL look at this as a current booking not a new booking when we check this on live availability. I have asked again if they have had any cancellations for the current cabin that you are in to see if we can then get this under the price match but again still sold out, so I will keep trying as best as I can".
So, it now appears that I have to be satisfied with waiting until my TA decides to contact RCCL again (which I doubt will happen as they always have to tell me how busy they are and how long it takes to get through to them) on the off-chance that there happens to have been a cancellation, which can then be used to calculate the refund/OBC.
Does anyone have any advice (other than never using this TA again?)
Duncan