Maris Otter is my favorite base malt--obviously, you were picking up on the MO vibes I was sending out
It just so happens that I split a fresh bag with two other brewers this week. Great choice.
I would have loved to have tried the mild ale malt; I can actually get that at my local HB supply. Many of the bigger/better supply shops in the US have it now, as well, so it would be a possibility for pretty much anyone to get it here. That said, MO is fantastic and gets my wholehearted vote.
Rather than ABV (at least at this stage) we should be focusing on OG. For example, I brew 5 gallon (roughly 19L) batches, and my system is capable of hitting 80-84%. Going by your volumes and efficiency, that equates to an estimated OG of 1.055, by the contribution of the base malt alone. Is that what you're seeing? Ultimately, we'll have to design the recipe based on ingredient percentages, I think--I'm not sure I could get 70% if I tried!
Hopefully knowing estimated OG will help each participant make their suggestion for each additional ingredient with a little more knowledge of the beer as it forms.
(Now I'm just hoping someone says "brown malt!")