went there about a month ago and the diversion was at j6 you go left onto the a45 and then off at the next exit and back round onto the a45 back towards the airport then off the a45 to the airport as normal
You're worrying way too much. Follow the standard route, and the diversion is very very clearly signposted. When you start to see the signs, follow them.
(Not meaning to sound patronising there chap, it really is extremely well signposted).
What time do you have to be there? I think I will avoid the M42 on Thursday.