Crazy Voyager is spot on.
Sleap will have the number if you can't find it yourself. Holmes Chapel is right on the edge of the CTR so you will need to call them. Although their main task is EGCC traffic, they're a friendly team up there.
Although it's been a while since I did it, I'm sure 99% of all ATCOs would prefer a phone-call regarding something you're not sure about, rather than just getting airborne and making a Horlicks of it.
Also keep a very good look-out around the southern end of the low-level corridor.
Have a safe trip.