Flexible working requests - carers and parents

Who is entitled to request to work flexibly?
What changes can an employee request?
Do I have to agree to a request to work flexibly?
How should an employee go about requesting flexible working?
How should I handle a request to work flexibly?
On what grounds may I refuse to grant a request to work flexibly?
If I accept a flexible working request, how long do the changes last for?
I am not sure whether I can accept a particular request – can I trial the arrangements to see if they work?