Hello Colin,
I hope I have understood your scenario.
switches in the middle must have the remote vlan configured and declared as remote-span.
switches in the middle should have:
an incoming link trunk or access port
an outgoing link trunk or access port
The overall path should be a point-to-point via source switch - middle switches - destination switch
this is because the effect of declaring a VLAN as a remote span vlan is to disable the MAC address learning function on that vlan so that all frames are carried on the RSPAN switched path (treated as unknown unicast) regardless of MAC destination address.
Ideally I think you should use one RSPAN instance for each switch.
OR
a single RSPAN if all access switches are connected directly to core switch but take care on what trunks the remote span vlan is permitted in order to build a point (core) to multipoint (access switches) but I'm not sure this can work I'm afraid traffic could be sent also from branch to branch and this would be a terrible mess.
Hope to help
Giuseppe