Just got the following message from Topcashback:
"After looking into this further for you, I can see that your claim has been declined for the reason:
"Transaction has yet to be validated"
which would suggest that they still intend to uplift your cashback to the correct amount during the validation period.
Can we please ask that you allow your cashback to progress to confirmed, and if, at this stage, it has not uplifted to the correct amount, do get back in touch and we will be able to advise you further."
So we just have to wait and see what happens with the transaction and then raise a claim if it is still the wrong amount once it is confirmed.
I'm pleased to say my cashback has been paid in full, although in 2 installments. I hope you guys get yours sorted.