Well, having spoken to both Jet2 and P&O, it appears passengers are being mis-informed.
Our booking states that if we wish to increase our luggage allowance, we must contact the airline. I did this and was first advised by Jet2 that it can be actioned directly with P&O. I disputed this, based on the information from the P&O booking. Then, Jet2 provided me with the portal address in which to achieve this. However, a tour operator number is required, i.e. from P&O. I did not have this, so I called P&O.
I spoke to someone after 32 minutes on hold. They tried hard but could not answer the question, and subsequently called me back later. They state (contrary to their booking information) that when passengers are travelling on a chartered flight, we are not permitted any additional luggage allowance. So, case closed (or not ! if you pardon the pun).