The brewer's description tells us ABV= 7.6%, malted barley, whole hops, well-water. We get a few more clues from this site:
http://www.shamburg.com/marstons.html, such as Maris Otter malt, Fuggles and Goldings whole hops, mineral-rich Burton water profile, some sulfur aroma. I didn't see any mention of unmalted roasted barley nor simple sugars. The recipe is supposedly 500 years old, so I doubt it calls for any fancy crystal, caramel or melanoidy type malts. The BJCP handbook lists Marstons Owd Rodger as a classic example of 19A English Strong Ale: Old Ale, which helps us dial in the missing stats. So, here's my all grain clone recipe:
6 US gallons = 5 Imperial gallons ≈ 27 Liters
14 lbs ≈ 6.4 kg ≈ 87.5% Maris Otter 2-row pale malt
2 lbs ≈ .9 kg ≈ 12.5% British brown malt 60-70L
1.5 oz ≈ 43 g Fuggles hops, boiled 60 minutes
1 oz ≈ 28 g Fuggles hops, boiled 30 minutes
1 oz ≈ 28 g Goldings hops, boiled 10 minutes
Single temp mash ≈ 65° C
OG ≈ 1.077, ABV ≈ 7.6, IBU ≈ 31, SRM ≈ 14
This is a classic big brown English ale, but should be pretty simple and inexpensive to brew. If you can't find (or don’t want to buy) Brown Malt, you can take 2 lbs of your base malt, soak it and bake it to make your own (this'll give you a wide range of bready, caramelly, toffee aromas, and will be caramelize unevenly similar to Old World practices.) You'll get best results if you know how to "Burtonize" your water (mainly by adding gypsum/calcium sulfate.) Optionally, add Irish moss near end of boil for clarity.
Yeast: 1st choice would be a historical strain linked to Burton, with low-medium attenuation but high alcohol tolerance, such as Brewlab Burton 1, Wyeast 1275, White Labs WLP023, BSI A-75 & BSI-23, among others. Since this beer is known for some sulfur aroma, Homebrew guru Charlie Papazian's yeast sold as White Labs WLP862 "Cry Havoc" might be perfect. You're striving for 7.6% alcohol, so use a healthy starter, aerate well, perhaps even utilize extra nutrients and rousing. Don't be afraid to ferment warm, fruity esters are desirable here.
At bottling, prime with ½ cup brown sugar from your kitchen, boiled with a little water to dissolve and sanitize.
Cheers!