I just wondered what the current rules were with regards to formation flying. I know that it is permitted as long as each aircraft commander that is taking part in the formation agrees beforehand to partake in it.
Yep.
Also, I guess you have to file a flightplan stating that you will be travelling in formation?
No requirement.
Also, when arriving and departing an aerodrome, are aircraft allowed to depart and land in formation (ala the Red Arrows) in controlled airspace or would ATC insist that only 'one at a time' would be allowed into and out of the airfield?
One at a time is the norm hence the run and break.
Formal exemptions can be granted if required. For example, Hutton Aviation get one at North Weald for the formation school.