Happynappies there are all sorts of things wrong here!
Firstly obviously your employer can accommodate your original request, that is clear.
Secondly, if they are penalising you for changing your hours following a flexible working request by changing your terms and conditions like this that's discrimination. See here about the rights of part-time workers and here about sex discrimination.
Thirdly it is illegal to put someone on an 'hourly contract' and remove all their employment rights. 'Job security', your rights to notice, equal pay, benefits etc are not things an employer can take away whenever they like by calling a contract 'hourly'. See this interactive tool to work out what your basic employment rights are.
It's good you have a union to advise you, do use them. It sounds like your employer may not be currently taking legal advice about this, so i think they are probably bluffing and hoping you will give up through the stress of it all. My guess is if you put in a tribunal claim they will need to take advice and will suddenly back off. So although you are asking for people's experiences of tribunals, I would be astonished if this claim actually made it as far as a tribunal.
Ask your union if they have any contacts with employment lawyers, see if you can find one - they will often give you a free consultation initially and listen to your case and advise you whether you have a good one. You could also visit the CAB who should be able to advise you.
Bringing a claim is stressful, but I think as i say, this is highly unlikely to make it as far as an actual tribunal and I really think it would be worth your while in this case to put in a claim form to show you are serious, this will hopefully resolve the situation and reduce the stress you are under.
Whether you feel happy to work for this employer in the long term is obviously a different question you will need to think about.