The charitable trust must comply with its governing document and charity law. It cannot support the Parish Council's finances, as the money belongs to the trust and not the Council. It must be kept seperate from the Council's finances and will need to have its accounts examined/audited in accordance with the Charity Commission's requirements, which vary according to the legal structure and annual turnover. Details on the Charity Commission's website.
The Charity Commission and the Local Government Association have published a simple guide to the key issues likely to arise in this situation. Search for "Councillors' guide to a council's role as charity trustee."
Depending upon the aims and objectives of the trust, it may be possible for the trust to pay for things for which the Council would otherwise be obliged to make provision.