In the absence of any answers, I'm going to suggest the following grain bill based on the known ingredients and have it shouted down
In my defence, should I need it, I am in a hotel room in Tokyo without access to Beersmith!
Forget the ale
90% Maris Otter
1% Dark crystal
4% Biscuit
1% Caramunich
4% Munich
Now I sit back and wait ...
FWIW, I make an excellent (if I do say so) bitter with a grain bill of 96% Maris Otter, 2% Simpson's dark crystal and 2% Simpson's medium crystal.
Don't overcomplicate things. Sometimes - if not usually - simple is best.
Let us know what you decide and how it goes.
Cheers,
Oliver