Actually it is a whole lot easier than you think.
If your paragraphs are setup with automatic numbers like this...
4.2 a heading
(a) the first clause in 4.2
(b) the second clause in 4.2


(c) the third clause in 4.2
(i) the first subclause of 4.2c
(ii) the second subclause of 4.2c

then you put your cursor where you want the cross-ref and go to Insert > CrossReference: Reference type = Numbered Item; Insert Reference to = Paragraph Number (full context), and choose the ii paragraph and click the Insert button.

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Andrew Lockton
Chrysalis Design, Melbourne Australia