Look, clearly Mrs Lawrence is not going to win because ...
1. There is no contractual relationship.
2. Neither "consideration" nor "intention to create legal relations" are present.
3. The boy who was invited is a minor, so does not have legal capacity to be bound by the agreement.
4. Mrs Lawrence has not suffered any loss. Presumably she had to pay the fifteen quid, whether the boy turned up or not, so she is not out of pocket.
However, it's an important principle that you don't accept an invite unless you're free and, once you've accepted it, you do not book something else on top of it. Otherwise, you turn into one of these people who will let you down if they get a better offer.