Following your advice
Created location constraint
location l-drbd01 ms-drbd01 3000: sn03
This ensures the M/S resource runs on the designated host
Master/Slave Set: ms-drbd01 [drbd01]
Masters: [ sn03 ]
Slaves: [ sn04 ]

then use the cli to migrate
sn03:~ # crm resource migrate ms-drbd01 sn04
INFO: Move constraint created for ms-drbd01 to sn04
- switched the roles from M/S to S/M and auto created a prefer location constraint ( with infinity) overriding the manuel location contraint with a score of 3000
Master/Slave Set: ms-drbd01 [drbd01]
Masters: [ sn04 ]
Slaves: [ sn03 ]

now 2 location constraints
location cli-prefer-ms-drbd01 ms-drbd01 role=Started inf: sn04
location l-drbd01 ms-drbd01 3000: sn03

HAWK unmigrate moves the resource back (and auto deletes the prefer location constraint)
or deleting the location cli-prefer-ms-drbd01 moves the resource back on sn03 (either through HAWK ir cli)

-- cli unmigrate
crm resource unmigrate ms-drbd01
INFO: Removed migration constraints for ms-drbd01
-- cli delete prefer location constraint
crm configure delete cli-prefer-ms-drbd01

So this works in keeping the replication going on the 2 hosts.

Thank you for your help.
Hope this helps others.