Finally found a fix for this. Adding a route as suggested by @corradolab was unnecessary as it turns out. This problem was irking me to no end as all other traffic was working well except SMB. I'd tested HTTP, FTP, ping e.t.c. to the LAN and all were working fine - just not SMB, and only SMB on Windows (macOS clients were fine).
I thought I might be running into this bug;
https://redmine.pfsense.org/issues/8964
But it actually turned out to be something in the way Windows authenticates to the server. To fix, you need to go to the Windows Credential Manager and add in the credentials for the SMB server before trying to connect.
After that it all works fine.