Betfair posted the following message on their forum.
Please be aware that the off of the 16.45 Southwell was missed. The market was never placed in-play and the following Rules and Regulations apply:
If a market is scheduled to be turned in-play but Betfair does not suspend the market at the time of the ‘off’ and the market is not turned in-play at any time during the event, all bets matched after the scheduled time of the ‘off’ will be void (in the case of horseracing and greyhound racing, bets will be void from the official rather than the scheduled ‘off’ time).
The BSP will be manually reconciled before the market is settled.
We apologise for any inconvenience this has caused.