Hurrah!

Success at last.

Several days of tapping, banging the jar on the worktop, levering and channelling of all my frustration came to naught, but then I remembered. Heat. That's what was needed. A little light expansion.

Half an hour seated on the radiator (the jar, that is, not me you understand) did the trick. And I am delighted to announce that ...
we now have pickles!

(The heater is also now in the house, although that did require some assistance).