/* Keep the entire webform block centered in the main content area */
#block-pcse-webform {
  max-width: 960px; /* matches GOV.UK/NHS content width */
  margin-left: auto;
  margin-right: auto;
  padding: 0 1rem;
  box-sizing: border-box;
}

/* Now control the form layout inside the block */
#block-pcse-webform form,
#block-pcse-webform .webform-submission-form {
  max-width: 600px;  /* your constraint */
  margin-left: 0;    /* left align INSIDE the content area */
  margin-right: 0;
  box-sizing: border-box;
}

/* Keep children contained */
#block-pcse-webform * {
  box-sizing: border-box;
}