A fairly difficult first question.
With contract law, I think you have to be very systematic. To sue someone for breach of contract you need to show four things: (1) that a contract existed (offer/acceptance, certainty, consideration, intent to create legal relations), (2) that there is a relevant term of that contract, (3) that the contract has been breached and (4) claimant has suffered loss. If you are in doubt about how to structure an answer, I would stick to going through these four aspects.
You could structure as follows:
Existence of a contract
- Was there an offer?
- Could the offer be revoked once performance had started on 24 March? If so, was posting a notice on the website on 28 March valid revocation? (first point of the letter)
- In case the offer was not revoked on 28 March, was it enough for Paul to find out through the manager, once performance had already been completed?
- Was there acceptance? How long did Paul have to post his card, did he have to make sure it reached head office before 2 April? (4th point of the letter)
- Was there certainty, consideration and intent to create legal relations?
Terms of the contract
- Did the number of lengths have to be completed in one attempt? (second point of the letter)
- Did Paul have to swim exactly 300 lengths and no more? (third point of the letter)
Breach of contract and loss
- If there is a contract and Paul complied with the necessary requirements, what is Paul's loss? This bit is simply, its just one year's membership.
As a general rule, make sure each paragraph is IRAC. State the Issue, state the relevant rule of law, apply to the facts, conclude.