Hi PP. The reduced night time readiness originated with the introduction of the SK those many years ago. Indeed I was in the crewroom at Lossiemouth when it was being discuseed and implimented. Remember that the previous Whirlwind (and Wessex) generation could not operate in all weathers and only with some difficulty at night and there was no nominal night high readiness. Quite simply it is a requirement generated to permit the SAR standby crew to maintain a nominal 24hr standby shift with a suitable period of sleeping watch to minimise fatigue. This extended night readiness also allows the crew to awaken properly, and better assess the actual conditions and flight plan before launch - always neccessary in the greater risk conditions at night. Originally each RAF SAR flight would go down to 45 at sunset whatever that time was but this was fixed at 2100 in the 1990s when the total number of UK SAR flights were reduced and it was felt that a more consistent UK night time coverage and readiness approach was required. This does not exclude the RCC rerquiring an individual Flight maintaining higher readiness should the "local" forecast activity require it - although this has seldom been implimented.
Cheers