Googling the St Austel Brewery web site, it confirms they use local grown Maris Otter Pale malt and Chinook, Cascade and Willamette hops. Maybe Anglo American origins but the bottle does say "Cornish IPA" on the bottle

I contacted St Austel to get some info, I informed them I was a homebrewer and they were very helpful. This is an extract from their reply " I can reveal that Chinook and Willamette go into the boil, Willamette, Chinook and Cascade go into the hop back and Chinook is used to dry hop. IBUs are around 50."
Looking at a 23 L batch, can anyone give me some help on :-
How much Maris Otter to hit 5.5%., plus what proportions and amounts of hops to use. I know the hop question is tricky but any ballpark amounts to use, from anyone familiar with these hops.
My homebrew shop in Plymouth has the hops and he also sells Mangrove Jacks M42 and M44 yeasts. Any thoughts on yeasts? This will be my first attempt at an IPA. Thanks in anticipation.