Tax Charge on Distributions from Settlor-interested Discretionary Trust?

I have a settlor-interested discretionary trust. The trust’s only asset is an investment bond and so the trust receives no income. (Periodic withdrawals are made so there is occasionally income tax due on a chargeable event, but the funds remain capital in nature.) Monthly distributions are made to the primary beneficiary (who is disabled) with the intention of providing him with a source of income. As this is income in his hands, my reading of ITA 2007 s493 et seq. is that it should be treated as an income distribution by the trustees and thus gives rise to a tax pool charge (which the beneficiary can potentially reclaim). Of course, normally the settlor would have already been assessed on the trust income so there would be no charge when it is distributed - but in this case there is no income and so he hasn’t been. HMRC do not seem to agree with me and have removed the charge from the calculation with no further explanation. I appreciate that settlor-interested trusts do not normally have a tax pool, but I can’t see anything in the legislation that specifically prevents s496 from applying to them. I would be grateful for confirmation of whether I am wrong and why.