The legislation governing the appointment of committees and sub-committees is contained in sections 101 and 102 of the Local Government Act 1972. There is no specific mention of terms of reference, however 102(2), requires the following:-
"Subject to the provisions of this section, the number of members of a committee appointed under subsection (1) above, their term of office, and the area (if restricted) within which the committee are to exercise their authority shall be fixed by the appointing authority or authorities or, in the case of a sub-committee, by the appointing committee."
So there is no statutory requirement, however, if you have adopted the NALC model standing orders, you will have resolved to determine formal terms of reference (4d in the current model).