Klm 695
Many thanks for the replies which seem absolutely sensible. Although I wasn't offered an alternative routing, may be I missed out on a free upgrade! The only thing I cannot understand is that the aircraft was an 747.400 which I assume is the largest seating capacity in the KLM fleet or maybe I am wrong. I do not have the registration but it was marked Asia and may usually be used for the far east. Overbooking is a possiblity but it would have been on quite a large scale judging by the rerouting queue.
PS The flight was KLM 692, 695 was the outward bound flight - whoops!