Thanks Clubman. It's just that it seems odd that BAEC want to charge an extra £2100 for the privilege of using them. Even if I paid the £200 change fee then it's still cheaper.

The other option would be to purchase the international on 1 ticket and internal on another but if I get hit by a delay (which is not unheard of in the Subcontinent) I'll be stung for the cost of another flight.