Your solicitor will know the words to use, but here is a suggestion:
"I bequeath to Habitat for Humanity Northern Ireland (Registered Charity Number NIC103240), Riverside Centre, Young Street, Lisburn, BT27 5EA, <insert gift type - see examples below>, to be applied for the general purpose of the charity shall at its absolute discretion decide, and I declare that the receipt of the Honorary Treasurer or Company Secretary shall be sufficient discharge to my Executors."
Choose one of the gift type examples:
