Trustees - TCGA92/S86
The trustees have to be non-resident for the whole tax year. Unlike individuals trustees can still change their tax residence during the year for the years 2013-14 onwards. If the trustees are resident in the UK even for one day section 86 will not apply and the trustees themselves will be chargeable on gains accruing at any time in the tax year.
UK resident settlor interested settlement
If the trustees became UK resident before 2008-09 it is likely that the gains would be taxed on the settlor under TCGA92/S77, see CG34700, and not on the trustees. TCGA92/S77 was the equivalent of section 86 if the settlor had an interest in a UK resident settlement. The list of beneficiaries that give the settlor an interest in a non-resident settlement are much wider than the list of beneficiaries that gave the settlor an interest in a UK resident settlement. A UK resident settlement that was outside the scope of section 77 may be caught by section 86 if it becomes non-resident. Section 77 was repealed in 2008.
Section 86 also applies if the trustees are UK resident and at the same time are treated as non-resident under a double taxation agreement. In this case section 86 applies only to the gains on assets that are protected by the tax treaty, see CG38530. The trustees themselves will be liable to capital gains tax on the gain on any non-protected assets.