The first GBP40 vouchers have arrived
2 days after two friends (me@hotmail and me@live) made a booking with the GBP20 voucher, the GBP40 vouchers showed up in the referer's mailbox (me@yahoo).
The good news appears to be that there is no restrictions on the GBP40 voucher, I used it with the "friend" account, and with the same credit card the "friend" used.
Total savings now: £140 for summer travel