On a charter flight (flight numbers starting MON) it can sometimes be the case that an insufficient number of people have paid to occupy all of the available legroom seats. If this happens and the aircraft is full then there can be a slight chance that you will be seated in one of those legroom seats without having to pay (since the only seats left for people to occupy are those seats with the additional legroom). However, if you want to get guaranteed legroom then I would not suggest this method.
Anybody wishing to sit in unoccupied legroom seats when they are onboard can do so for an extra cost. The prices at time of posting are as follows:
Charter (MON)
Short haul - £15 per sector
Mid haul (Canaries, Cyprus, Turkey) - £25 per sector
Long haul (includes Egypt and Jordan) - £35 per sector
Scheduled (ZB)
LEI/IBZ/MJV/ALC/MAH/BCN - £15 per flight
AGP/FAO/PMI/GIB - £17.50 per flight
LCA/TFS/ACE - £25 per flight