Response from NSS - Any GP, including Sessional GPs, are entitled to an NHSmail account. The issue is whether they should be attached to the practice they work in or not. I would suggest that if they are likely to work in a practice for 3 months or more they should be attached to the practice and if they move to another practice they are marked as a leaver from that practice and then added to the next by the practice admins. This would need to be done by your local administrator. If they are only working for a week here or there, they should float within the Board (not attached to anything except GGC Board).
To apply for an NHS Mail account phone the NSS ISD Helpline on (0141) 282 2100, and log a call. Tell the person on the help line that you are a Sessional GP looking for an NHS Mail account and which Health Board you are hosted by. Your application will then be actioned.