Pregnant gain

The trustees of a discretionary settlement made capital appointments of shares in an unquoted non-trading company to two adult beneficiaries in 2001-02. Large pregnant gains were deferred by holdover claims which were competent.
In 2005-06 one of the beneficiaries went to work overseas on a two-year contract such that there is the potential to assess the holdover gain on her if as now seems likely she extends her contract such that she will have been neither resident nor ordinarily resident for at least three years from the date she first became non resident. In these circumstances the protection afforded by TCGA 1992 s 168(5) will cease and the gain will then become assessable on her.
My query is as to the mechanics of assessment in these circumstances. Section 168 refers to assessment but we are now in the realm of self assessment. Is there a...