Some people think that a parish council cannot create a working party, and that any group set up by a council has to be a committee or sub-committee.
Personally, I don't take that view, instead following the general principle of English law that something is lawful unless it is prohibited. There are certainly times when a working party seems more appropriate than a formal committee. The council can't delegate decisions to a working party, but can ask it to report back so that the council can then make decisions based on the results of the working party's efforts.
The question of how the working party should operate is then all about what is going to work. There could be times when a totally unstructured debate is useful, but generally all meetings work better if someone acts as chairman to keep the discussion on track.
Early on, a working party might have no agenda - although it obviously needs some subject matter. It's likely that as it progresses, it will be more effective if it decides what its meetings are trying to do - and that tends to require an agenda.
Basically, if it is a working party, there aren't any rules, but standard practices for running meeting have become standard because they work!