The definition of a disposable pecuniary interest includes "Any employment, office, trade, profession or vocation carried on for profit or gain, which you, or your spouse or civil partner, undertakes" I would argue that being a trustee amounts to "office" as quoted above.
If you receive a salary or cash allowance, or the charitable organisation as a whole under your actions generates profit (regardless of what is done with the profit) I would advise declaring it.