Bad Batch - Need Help

I recently edited a recipe for an English Bitter that I found on another forum. I changed a few of the ingredients but kept the basic process the same. I just tasted the beer and it came out wrong: The beer had a very watered down and strange taste that is nothing like it was intended to be. Also the carbonation did not turn out at all.

This is my exact procedure. Maybe one of you could tell me some reasons my batch came out wrong.

Information

Beer Style: English Ordinary Bitter
IBU: 40
Batch Size: 6 USG
Initial Gravity: 1.040
Final Gravity: 1.012
% ABV: 5.0
Primary Fermentation: 5 days
Secondary Fermentation: 5 days

Ingredients

Yeast: London Ale III (1318)
Malt Extract: Muntons Amber
Hops: Kent Golding Hops (5.4 AAU’s), Northern Brewer (8.5 AAU’s)
Grains: Pale Malt (Maris Otter 3.0 SRM), Cara-Pils/Dextrin (2.0 SRM), Crystal Malt - 60L (60.0 SRM), Chocolate Malt (450.0 SRM).
Extras: Whirlfloc Tablets, Gelatin Finings

Mini-Mash Method:
Prepare the ingredients for the mash

Pale Malt, Marris Otter 7.26 lbs
Cara-Pils/Dextrine 0.55 lbs
Crystal Malt 0.25 lbs
Chocolate Malt 0.06 lbs

Add 1.5 gallons of water to the stock pot. Heat the water to 75*C (167*F). Add the grain bag and let it soak for 60min. Be sure to leave the grain bag loose so that the water to flow through all the grains.

Add the Northern Brewer hops to the stock pot prior to sparging.
Batch Sparge 1: Sparge with 0.5 gallons of water at 77*C (170*F)
Batch Sparge 2: Sparge with 0.5 gallons of water at 77*C (170*F)
Add water to the stock pot until the total volume is 3.5 gallons and let boil for 60 minutes.
The estimated pre-boil gravity is 1.030

Start the boil.
1.) Once the wort is up to boil, add the Muntons Amber malt extract.
2.) After 15 minutes add the Kent Golding Hops.
3.) After 50 minutes add one Whirlfloc tablet and turn off the flame.

Cool the wort to the fermentation temperature of 70*F. Siphon the wort into the primary fermenter and add water until the final volume of 6 gallons is reached. Be sure to aerate the wort by either rocking the bucket back and forth for 15 minutes (3, 5 minute intervals) or by continuously transferring the wort between 2 buckets until the head is too high that it might overflow.

Add the London Ale III yeast to the primary fermenter and close the lid. Allow the wort to ferment for 5 days in the primary fermenter. After the 5 days, add the corn sugar and bottle the beer. Carbonation will take 5 days. After this time it is recommended to age for 2 weeks at 17*C (63*F).

You need a bigger boil if you plan on doing 6 gallon batches again. Doing a 3.5 gallon boil is not enough. By the time you got done boiling, you probably had around 3 gallons, if not less. Then adding 3 gallons of water to that... I'm not suprised it tastes watery. It's extremely diluted.

nitial Gravity: 1.040
Final Gravity: 1.012
% ABV: 5.0

Click to expand...

Something is wrong here. A 1.040 OG will not give you a 5% beer. More like 3.5%, if your FG was 1.012. The hopping won't give you 40 IBUs either. You've got 6 gallons of Dark Mild.