Thank you to everyone who responded to my gherkin SOS. It has taken nearly a week, but I am happy to announce that we have finally got it open. I tried all your suggestions, but in the end it was simple. I went out for a night of sewing, gossiping, eating gingerbread and drinking tea, while the Boy Wonder had some friends over for sausages, beer and a violent DVD.
When I came home, feeling a little like someone from an Austen novel, the house was dark and silent except for the whirring of the dishwasher. This heartwarming note lay on the kitchen counter:
And they say romance is dead! It turns out that eating rather a lot of spicy beef sausages from the halal butcher, drinking several French beers and watching a movie about a boy who wants to be a superhero gave the Boy Wonder just the strength he needed to wrest open the lid. I'm happy to hire him out if you have a similar problem. Just be aware that he can eat quite a lot...