There seem to be at least two approaches that could be tackled immediately, one requiring councillor involvement and one not.
Council meetings (including committees) must be held in public and be based on an agenda that has been made public several clear days before the meeting. It's therefore possible for anyone to attend a meeting, armed with details of what matters can be decided, and to make notes on the decisions made. There is no requirement for minutes to record the discussion, but all decisions must be recorded. This last point is true even for items where the public is excluded, something that requires a formal vote and a reason. Minutes must be ratified either at the same meeting or the next one, and electors have a right to inspect them and to take copies. That opens up the possibility for anyone to check that decisions are made on matters in the agendas and that they are correctly recorded. If they are not, options such as formal complaint to the council, writing to the press, etc are available.
In relation to the behaviour of the clerk and particularly the management of staff, the council employs the clerk. The council has an obligation, as employer, to manage the clerk including periodic appraisals. Arrangements must be in place for a councillor or group of councillors to act as the clerk's manager, and those arrangements must be approved by the council (or possibly a delegated staffing committee). It is up to councillors to see that problems with the clerk's work are dealt with constructively.
Individual employees also have rights, and could raise a grievance, and ultimately take the matter to an employment tribunal if not resolved. However, individuals may be reluctant to embark on this process for fear of losing their jobs, and the outcome of any legal procedure is never entirely certain. Staff could consider involving their union, if any, or joining a union if they are not currently members.
In the end, the most effective remedy is public pressure and the ballot box. Councillors will all be up for election next May in the four yearly local council elections. So now is an ideal time to create public pressure for reform.
In relation to external support, if there is actual impropriety it may be possible to involve the senior legal officer of the district council. The district council does have some responsibility for the proper running of local councils in its area. The other common route for support is the National Association of Local Councils, of which many councils are members, through local associations. They are likely to require that matters be raised by either the clerk or (if the clerk is part of the issue) by the council chairman.