Put the lentils into a pan of cold water, add half of the celery and the bay leaf. Bring to the boil and cook for about 20 minutes until the lentils are tender. Drain, rinse and drain again. Discard the bay leaf and celery.
Meanwhile, finely chop the gherkin, spring onion and the other half of the celery, then scatter them into a salad bowl. Add the mayo and mustard. Mix well.
Add the drained and cooled lentils, toss, then stir in the parsley. Season to taste, then add the cider vinegar. Taste, adjust the seasoning, mustard and vinegar.
Serve at room temperature.