The Supreme Court has recently decided that in cases of dismissal on notice by post (where there is no contractual provision governing when notice takes effect) the contractual notice only starts to run when the letter by which it is communicated comes to the attention of the employee and they have either read it or had a reasonable opportunity of doing so. It was decided that that there is a term implied in all employment contracts that written notice only applies when it is read (if the contract does not spell out something different)
Giving notice of dismissal in person, if possible, or at least verbally, is always the clearer and safer option, which should then be followed up with a letter and/or email. Finally, always ensure that comprehensive records are kept of the date and time of any such conversation and exactly what was said.