Hell in the middle was at Cannock Chase this year, it had a lot less mud than Delamere due to the nature of the ground there, however it was all entirely iced over so instead of slipping in mud we were slipping on ice!
The next Hell in the Middle is at Trentham Gardens in 2013. As far as I'm aware it's the first time they've held it here. I assume the route will mostly be in the wooded area outised of the public gardens (as they're far too pretty for us to come clomping through!) If it is in that area then it is very muddy and about as hilly as Delamere. Great shops for pie and cake at the finish though!
It was definitely not "too extreme", the 2 bogs of doom were great fun, and it was actually quite a mild day so not too cold if you kept moving. I did Hell in the Middle earlier in the year as well; it was well below freezing that day so one of the water obstacles was sensibly cut out.
The Hellrunner series are all about the mud, bogs, hills and water - if you're not up for that then enter a road race!