I’ve had a search but can’t find a definitive answer.
I have an underlying IHG masterdcard credit (which I can’t find to get the card number off…) that I’d like to transfer the balance to a new Santander CC (0% balance transfer fee).
Can I just enter the transfer details of my curve card via Santander and it will effectively credit whichever card is selected in Curve?
This wouldn’t have any associated fees right?
I really wouldn’t advise this. Santander probably won’t allow it, since they can see you’re not transferring a credit balance, if you give them a debit card (ie Curve).
You’d be better off getting a money transfer to a bank account and paying off that way, if Santander are offering that?
If you’ve lost your card (details) just report it lost and get a new one issued.
If Santander did allow this, there have been several reports here of Curve doing card credits wrong, and treating them as charges - ie you risk getting debited rather than credited. Since you’d be bypassing Curve Fronted fees (their fee for paying one credit card from another), so they may not be happy even if it did work as you hope.
Doesn’t this all sound like a lot more hassle than just getting a replacement card? 
Yeah fair enough. It’s a pretty standard money transfer so wondered if it would work.
The card is somewhere, I just didn’t want to look for it as I thought the point of curve was to replace having multitudes of cards. 
Yes for making normal payments but it’s not really designed for balance transfers to underlying cards, that goes directly between the card companies! I take it you don’t have it saved in your Google Wallet, etc (eg from Chrome)? That would give you the full card number.
No - because credit card providers processing Balance Transfers simply lookup the BIN of the card entered against a list of bank account numbers and then send a bank transfer.
You will need to locate the number of your underlying card and apply for the BT in the normal way.