wiggy - I've been NATS qual'd since 1990. There's always the chance for exceptions, which is why we have human beings instead of silicon chips running the show.
Had a developing slow speed incident at altitude due to temperature spike to +29C at FL380(!!!!). VHF has unuseable due to static. Immediately selected 'lower' on TCAS to sort traffic if we had to do a comm out emergency descent. Luckily we flew out of the hot air bubble prior to having to descend.
Thirty minutes later another flight was not as lucky and had to descend 4,000' as they reached minimum cruise speed due to the temperature spike.