5. Where under these Rules any document is to be served on an individual other than the court administration officer it may be served—
(a)on the individual personally;
(b)if he is subject to service law, by post in a letter addressed to him at his unit;
(c)if he is not subject to service law—
(i)by leaving it at his usual or last known place of abode; or
(ii)by post in a letter addressed to his usual or last known place of abode.