Believe me there are lots of examples where Royal Mail have bypassed more senior part timers for the opportunity to become full time.
One of the things I am currently involved in is attempting to make FTCs permanent and there are quite a few who have been doing full time hours against a part time contract and this opportunity has not been offered to other part timers in the office some who have over 10 years service.
Equally Royal Mails recruitment is a shambles you have an office recuit someone on a 25 hour contract and then a few months later recuit a FTC on a full time contact when they should be going back to either a permanent part time member of staff or to the 25 hour person.
The opportunity for part time staff to become full time is getting harder as Royal Mail now have a policy that when a full time duty becomes vacant they only fill with a contract of 25 hours unless agreed by the delivery director otherwise.
This is why all opportunities for part timers to become full time has to be done fairly and in line with the way forward agreement. Not simply because a manager has made a mistake and not offered an opportunity to increase a part timers hours outside of the agreed pecking order.