Cost Sharing
The requirement is that the pilot must pay his proportionate share of the cost.
eg If there are 4 POB including the pilot, then the pilot must pay at least 25% of the cost.
There is no requirement that the cost is divided equally between the persons on board. The passengers may divide the remaining cost as they wish, or one may pay for all three passengers.
Private or Public Transport?
The ANO provides a number of very complicated tests and exceptions to determine whether a flight is private or public transport.
In the context of Paul 101's question -
Article 130 provides that (with specified exceptions) "an aircraft in flight shall for the purposes of this Order be deemed to fly for the purposes of public transport if valuable consideration is given or promised for the carriage of passengers or cargo in the aircraft on that flight."
If 'remuneration' was the test for determining whether a flight is a private or public transport, it would be easy to apply. However, nothing in UK aviation legislation is ever that easy to understand or apply, so the pilot has to ask himself whether 'valuable consideration' has been given or promised. To assist him, the ANO provides a helpful definition:
'Valuable consideration' "means any right, interest, profit or benefit, forbearance, detriment, loss or responsibility accruing, given, suffered or undertaken pursuant to an agreement, which is of more than a nominal nature."
This definition is not only very difficult to understand but extremely wide - far wider than remuneration.
I can't vouch for the accuracy of the following example (I wasn't involved in the case) but I'm told the owner of a flying school was prosecuted for illegal public transport in these circumstances:
He agreed to fly some local 'celeb' to publicise a local event, providing both the helicopter and his services completely free of charge. He was prosecuted by the CAA for illegal public transport on the basis that, because the school's name was on the side of the helicopter, he obtained 'valuable consideration' in the form of publicity in the local press and on the local tv station.
I'm told he balanced the costs of fighting the case and paying the CAA's costs if he lost the case against the cost of just pleading guilty, paying the fine and a smaller sum in costs - and decided to plead guilty.