If you are using a TTF that is valid for the last 10 mins of the flight (for the sake of argument in this case) are you legal?
According to AIP, you need to have a forecast that is valid for at least 60 after planned ETA.
1.2.5 A pilot in command must ensure that the forecasts cover the period of the flight and that the aerodrome forecasts for the destination and alternate aerodromes, to be nominated in the flight plan, are valid for a period of not less than 30 minutes before and 60 minutes after the planned ETA.