Reality check
If you are running a company which is suffering from previous bad/weak management and belligerant/militant trade unions, the situation you are in must be one where current working practices are very restrictive, costs/salaries are high, the company is overstaffed and the trade unions are making/interfering with management control/decisions.
The only way forward is to restructure the company in all aspects - for this you need the understanding and assistance of the unions.
If the only response that you receive from the unions is threats, demands, and then a strike, what are the ways forward left open to you - surrender or fight.
Regardless of the legalities, fire the striking staff, offer to rehire - but cherry pick those that are - good at their job - not militant unionists, once employed remove the restrictive working practices and increase the wages, the cost savings will pay for it, allocate an amount to meet legal costs and awards to those staff who are not re-employed.
Cull your management team, don't use them on any union negotiation as they have already lost any authority on previous situations, promote new blood from within (if its available!), hire new human resources manager and director, new 2nd and 3rd I/C.
And so it goes on ..............