I was looking into the situation for someone who was considering leaving RM next year & asking if he could keep his shares with Royal Mail. I showed him the information depending on length of time you have held the shares.
Also from the guide;
If you leave more than five years after being
given the Free Shares, you may instruct the
administrator, Equiniti, either to sell the
shares or transfer them to you so you continue
to hold them outside the SIP.
As you say Robert I now see, that for anyone remaining, one option is to keep the shares in the SIP.
Can I keep my shares in the SIP indefinitely?
After five years, you can leave your Free
Shares in the SIP for as long as you wish.