Hi, We have a second office which connects over a leased line VPN, when users from the main office go there and try to access their mail, personal folders etc they cannot see them.

There are many things to check here... Do you have connectivity to host that pst is on? (ping) Can you copy files between client computer and computer hosting pst? Can you see the pst from Explorer/My Computer? Is there another MAPI client(outlook) using the PST? You might also want to consider how large the pst is compared to the VPN/WAN connection. If there is a question try connecting to an empty/almost empty pst.