Again, I don't think the FTSE future matters. As the T&Cs point out, you have to trade at their price. All SBs were showing similar prices, so if they cancel this trade they might as well cancel every trade (only if it resulted in a punter winning, of course).
"27. Manifest Error (1) We reserve the right to, without your consent, either void from the outset or
amend the terms of any Bet containing or based on any error that we reasonably
believe to be obvious or palpable (a “Manifest Error”). If, in our discretion, we choose
to amend the terms of any such Manifestly Erroneous Bet, the amended level will
be such level as we reasonably believe would have been fair at the time the Bet was
entered into. In deciding whether an error is a Manifest Error we must act reasonably
and we may take into account any relevant information including, without limitation,
the state of the Underlying Market at the time of the error or any mistake in, or lack of
clarity of, any information source or pronouncement upon which we base our quoted
prices. Any financial commitment that you have entered into or refrained from
entering into in reliance on a Bet with us will not be taken into account in deciding
whether or not there has been a Manifest Error.
(2) In the absence of wilful default or fraud by us we will not be liable to you for any
loss, cost, claim, demand or expense following a Manifest Error (including where
the Manifest Error is made by any information source, commentator or official upon
whom we reasonably rely).
(3) If a Manifest Error has occurred and we choose to exercise any of our rights under
Term 27(1), and if you have received any monies from us in connection with the
Manifest Error, you agree that those monies are due and payable to us and you agree
to return an equal sum to us without delay. "