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@Supersuperleeds I am Treasurer and trustee for a small charity. This is a local u3a branch - all u3as are charities. Are we small, yes, tiny, with a turnover of +/-£15,000. I'm not financially trained but ran the management accounts for my last employer, the Treasurer side of my role doesn't present any challenges.
As previously said as a trustee you are legally responsible for the charity. One aspect to be aware of is the new Charity Commission requirements. If your question is stimulated by being asked to become a trustee you should be aware of the following, especially if the charity is long established. It's possible small, long established charities may not have encountered this before.
Over the past two years the Charity Commission has introduced a requirement for all charities to have a number, 10 - 12, of written policies in place. For example Equally, Diversity and Inclusion, Health and Safety, Safeguarding, Grievance and Disciplinary. These are all pretty standard but if a charity doesn't have them in place writing a policy is a great deal of work. These have to be taken seriously as the person submitting the annual return has to confirm they are in place. As a trustee you would be responsible, along with other trustees, for ensuring such policies are put in place.