From reading these sentences, the reader can infer that Jack is uneasy or uncomfortable about his visit with Margaret, as he is "wriggling miserably" and struggling to talk naturally. This suggests that he is not at ease and may be feeling guilty or anxious about the situation.
Additionally, the reader can infer that Margaret is displeased or amused in her response to the invitation. Her "grim enjoyment" while watching Jack and the fact that she accepted the invitation "without showing any pleasure" indicate that she may be taking some satisfaction in his discomfort or is perhaps trying to maintain a composed facade despite her true feelings.