They do not like any check on growth. So they need water if it's dry etc
although I haven't found that a problem this year. To keep the curds white
you need to bend/snap a few of the leaves over the curd so they stay in
the dark. The variety does matter too, we prefer the spring ones as these
grow with no insect damage and no possibility of any meat amongst the
curd. (Winter Walcheren Amardo April is one such)

Did you use any fertilizer and what is the pH of the soil?

--
Hi Bob, No I haven't use any fertilizer and the pH is something else I've
got to learn about.
This is my first year on an allotment and been thrown in the deep end
with very little knowledge of gardening.
Overall I am very pleased with my results but have failed with a couple
of things, namely , cauliflower, onions and carrots, although when my
carrots failed e.g. very small and forked, I put in another lot (although I
was told it was too late) they came up fine and I am still bringing them
home.

Wally