Heat oil up in a chip pan or deep fat fryer to 375°F.
Peel the potatoes and cut into chunky sized chips. Rinse and dry thoroughly.
Fry chips for about 3 minutes until soft but NOT colored. Drain and shake well. Set aside.
Put some flour onto a plate.
Dredge the fish fillets in the flour thoroughly - this is VERY important, it stops the batter sliding off when fried! Leave the fish fillets in the flour while you make the batter. (Some recommend making the batter at least one hour before - we have found little difference - so make it before if it is convenient or make it now.)
Put flour, bicarbonate of soda, salt and pepper into a large bowl.
Add the beer gradually, stop when you have a thick coating type of batter.
Drink any beer that is left!
Whisk thoroughly until mixture is smooth and there are no lumps.
Add lemon juice or a splash of malt vinegar, if desired. Mix thoroughly again.
Have your plates, newspaper or whatever ready for serving.
Adjust deep fryer to fish frying temperature of 320°F (See comment below by Donna for a good tip here).
Take one fillet of fish at a time and, holding it by the tail or thin end (!), swirl it around the batter until well coated.
Immediately plunge into hot oil.
As soon as it has crisped up and set, add your other fillets one at a time, taking out the first ones as they finish cooking, about 6 to 10 minutes depending on the thickness. Place onto a tray and keep warm in the oven.
Turn up the heat setting on the deep fryer to 375°F again.
Cook your chips until golden and crisp.
Serve on plates or newspaper with salt, vinegar and plenty of homemade tartar sauce.