Strange I have posted a reply to this 3 times that seems to have got
lost in space, or bounced.
The reason it's bitter is because it's been pollinated.
F1 hybrid (All female) Glasshouse cucumbers were developed to avoid this
problem commercially but are Expensive.
Outdoor varieties and old varieties like Telegraph are open pollinated
and are sold by weight and not by the seed, they have both male and
female flowers.
When you look at the plant, the flower with the start of a cucumber
behind it is female, the one on a short thin stalk is Male.
In the "old days" we would go through every day or 2 and pick off the
male flowers.
With varieties like Telegraph, when they have been pollinated they swell
at the end closest to the flower and were known as "Bull nosed", these
were dumped because they would be bitter, and have developing seeds in them.
David @ a for once Sunny Swansea Bay.