I thought the basics were that a PPL(UK) could share the cost of any flight with the passengers as long as the PPL payed an equal portion ie 4 seats (inc pilot) the PPL pays one quarter of the cost. It would be ok for the passengers to buy the PPL a beer or a coffee at the end of the day but anything much more would constitute gain eg buying the PPL a plasma TV or giving them cash.
I dont think you can fly twin without CPL so that rules out and grey areas with 109's or 355's!
Question is what happens when a PPL gets the chance to pos the A/C for maintenance at zero cost to them. Does that count? can they take someone with them and drop them off en route or at destination?