Thanks good to hear. Tyres definitely need replacing as they've worn well from last year's riding. I'll ride my bike for a few months prior to the event so it's worth investing in something decent for that riding. Can't stand getting punctures so I just want something heavy duty that will prevent the majority.
Marathon Plus are definitely heavy-duty but sadly, I do have to agree that the ride is rather stiff. I've only ridden Specialized Armadillo that were worse.
You will nearly always be safe enough with the ordinary Marathon (Greenguard at the moment) or the more expensive Marathon Supreme. The ordinary Marathon basically isn't armoured as far up the sides which means that it can actually flex a bit more. The Supreme is the same like that, but the current model uses Schwalbe's latest OneStar compound instead of the Endurance compound on most of the range. Edit: both still have sturdy protection on the bit that usually contacts the road.
Personally, I ride the crap-infested cycle tracks next to the A10 and A149 most days on the even-less-protected Kevlar-based Delta Cruisers which are even more comfortable and even then rarely have punctures. I do have a Marathon Greenguard on Dutchie's back wheel at the moment because I really hate removing it.
I avoid Continental Gatorskins because they do seem to offer little protection against West/North Norfolk's flints, for some reason. Vittoria Rubinos seem good but aren't the most resistant and I lose track of Vittoria's name/range changes.
If the roads are rough (which is what I thought you meant by "bad road surface" until I read the post), fit the widest tyres you can without fouling mudguards - 28mm minimum, 37mm best - and don't overinflate them (look for the Bicycle Quaterly BQTireDrop chart). It was quite fun riding along with a sportive two years ago (before they deteriorated to the point where I won't even ride along now) and we reached the rough road from Ten Mile Bank to Welney and all the narrow-tyred road bikers were dancing around gingerly while I glided along on 37s and actually started to overtake some!
(Edited to complete two paragraphs - where did the ends go?)
I think a route would have to be strewn with glass, nails and burnt out cars to make me put Schwalbe M+ on for anything over about a 25 mile ride. They're great for toughness, but you're a better man than I if you can do a hundred on them. My Trek hybrid initially came with Bontrager Hard Case (they were fine), but I switched to Schwalbe M+ after the Bonty's wore out and I really noticed. They are heavy feeling, and dead, and as @Yellow Saddle says they do rob energy. I do keep them on that bike though as I do gravel paths on it mainly. 
