Are companies required to have an employee handbook in place or is the contract of employment sufficient?

I understand that it is perhaps preferable to have a separate handbook as then changes can be made when required without the need to alter employment contracts.

What other basic employment law requirements are there for small companies?

Also, what would be the recommendations for wording on inclusion in employment contracts for the following?

1. Retirement

2. PILON clause

3. Garden Leave

4. Termination without prejudice – based on the employee becoming bankrupt, of unsound mind under the mental health act 1983, committed of a criminal offence

Thanks in advance.