I also have 18 kph in my spreadsheet, which is just below what I normally ride 600's, but that speed is based on moving time for me. Looking at past events, I spend around 5 hours stopped for various things and usually finish around the 38 hours mark. Overall average is down to about 16 kph average which I also have in my spreadsheet printout to give me a finish time of 20:11, but all of my previous 600's have only had just over half to two thirds the amount of climbing as the Pendle 600. So realistically, if I do my normal ride, including about 3 hours sleep and 2 hours lost in checkpoints and other stops etc, I will only have 2 hours spare to do all the additional climbing which to me is cutting it fine. Looking at some of the timings for other riders who have done Hardknott and Wyreside on Strava, that section itself will take longer than 30 mins and up to 2 hours (Heaven forbid, those who did it in that time must have walked it all) itself, but them slow times depends on how fit those slower riders are in comparison to myself.
So sleep will have to go out of the window probably till maybe after the Lake District and I have a good margin of time in hand to finish.
The only good thing.....at the moment, no rain forecast