Please! "Oral", on the one hand, but "written" on the other. "Verbal" may be either, since it means "by words" and words may be spoken or written.

Contracts may also be formed by conduct - and indeed that is what hapened in Carlill. The plaintiff bought the smoke ball (from a third party retailer) and used it as directed.

The burden of proof in civil matters is the balance of probability. A witness is not essential - it simply makes discharging the burden of proof easier.

A contract cannot generally be cancelled by notice, whether it be written or oral.

But a contract must be sufficiently certain. The minimum would be date and fee, and it could probably be inferred that if no express terms were agreed about slots and expenses or subsistence or accommodation, then they would be "the usual".