- We are currently looking at implementing the Diary View at work however we require a global view, So it requires all milestones/tickets/activities to be shown on the one Diary (Not just individual resources). Is this possible as I can not seem to find an option that displays all events on the one Diary view.

No option is not available. Echh user only sees items he is planned on, or responsible of.

- Is there away to Manually input planned dates (Start plan date only etc), or can it only be done in Planning and calculating activities planning.

Not directly, as these fields are readonly.
But you can define planning mode as "regular between dates" and enter fix dates in validated one. They'll be copied into planned dates on planning calculation.