Just by way of update re booking with full Avios online at midnight and the calling in once the UK Centre is open. I did this today, booked 2 CW seats to Santiago (having booked the outbound last week) online at 2 minutes past midnight paying full Avios for two SCL>LHR in CW. I then rang in today (at 11.30am, not 6am) and within 5 minutes they had linked the voucher to the return booking and half the Avios used have already been refunded to my account.
Incredibly straightforward, the agent knew exactly what she was doing and all sorted in 5 minutes. Far easier than calling an overseas call centre in the early hours!