Well, as if by magic (maybe they are reading this forum ), I got an email reply in the past few minutes from Singapore Airlines. But what on earth do they mean about seats being held back by Heathrow Airport?

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"Thank you for your email.

We have checked the availability; please note that seats are available to
travel from London to Singapore on 12th January 2014, on flight SQ
317.However, at present, we are unable to select seats as they are under
the airport control.


Once you make the booking you may write to us with reference number and
seat preference, in order for us to update the request, on the booking. We
would recommend you to check-in online at www.singaporeair.com between
48hrs and 2hrs prior to flight departure, see what seats you have been
allocated and change the seats if required, as per the availability at the
time. As all seat numbers are not guaranteed until the time of check in and
may change due to the needs of the aircraft on your day of travel.


Thank you for choosing Singapore Airlines."

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