5% maris otter isn't going to have any really meaningful flavor contribution. It's just a slightly more flavorful base malt (and it does tend to contribute a bit more sweetness as a base malt versus pale/2row).
I like that you're working around something simple, though. I would bring up the amount of maris otter drastically (bring it up 50% or replace the pale malt with it entirely) if you're shooting for a nice malty body.
As for mash temp, hops can have a definite sweetness as well as alcohol, and with a high gravity of somewhere around 1.070 OG I would mash low and try and get very high attenuation and more alcohol. With a mash temp like 155 or similar you would more than likely end up with far, far too much body from low attenuation, particularly on top of a big pile of hops.