Extremely bitter beer

I agree with the other answers. The target AA was 2.7% I've seen Czech saaz range up to 6%. If you still have the package you can check.

Also keep in mind that good balance residual sweetness. If you're over attenuated, the bitter will be perceived more.

While beers can be very bitter to taste soon after fermentation they will mellow with age and become less bitter. I made an insanely bitter DIPA some time back and thought I had made a really bad mistake in brewing it. After 8 months it was considered one of my finest brews by friends who sampled it. So beers that are initially very bitter, perhaps too bitter, will become very drinkable with time. Modern, commercial beers are brewed to be consumed quickly but a well conditioned ale is a thing of beauty! It takes time. Few brew recipes mention the point and many drinkers seem to think it a crime to drink beer more than 6 months old. Wine drinkers, on the other hand....

If however the brewer thinks the beer too bitter then the hop bill must be adjusted to suit.And that adjustment must be as the ingredients change (eg harvest to harvest or batch to batch). If one is lucky to have 25Kg sack of hops in the freezer then it should be simple enough to run a few brews and get the required taste repeatably. But with different supplies or suppliers the bitterness/taste can "wander" around the desired point.

Were the hops used higher in AA than those used by other brewers? Were the hops fresher or older as age can affect the bitterness greatly? And indeed as others point out, liquor water higher in sulphates will give the beer that famous "Burton tang" accentuating the bitterness? Was any late or dry hoping used?

Maybe posting the formula/recipe and method would give and insight.

EDIT: I note that Saaz hops were swapped for sterling. That is the probable cause of the excess bitterness. Saaz are a "noble hop" with low Alpha acids (eg 2%). whereas Sterling are usually quoted as higher AA - eg 5%-6% Alpha acid. So these are not directly interchangeable, or not without pro-rata adjustment in quantity. I imagine half the amount of hop used would have produced more equal results to Saaz.

But as I said before - this hoppy bitter beer is entirely within the bounds of possibility. Bottle and leave to condition for a month or two (or longer). You will probably wonder why you even questioned its quality!

As indicated in the other answer, your primary culprit is probably the hops. Alternately, if you don't know your water chemistry makeup, you could be experiencing excess sulfate - your sulfate-to-chloride ratio has a very heavy effect on the perception of your hops and malt.

I strongly advise reading John Palmer's article about brewing water, or even better, buying the book. If you don't have a water chemistry setup, homebrewtalk has a lot of water profiles that people have posted - yours might be one of them.

And finally, you can always send a water sample to Ward Labs to have it analyzed. With a good knowledge of your water chem, you should be able to isolate any other problems you have with taste and Ph.