Welcome to the forum. Although you can post in any forum, the USA forum is here in case of local regs or laws
Managed to finally unscrew it and replace it, thanks for your help! Now in the GF’s good books as she can finally have a bath and not a shower 😂There are two flats near the top of the defunct mechanism which you should be able to grip with an adjustable spanner, and free it to unscrew. If that fails try slip joint/ gas/ water pump/ whatever you call them/ pliers. You may need brute force!
Reply to the thread, titled "Pop Up Bath plug sticking" which is posted in Bathroom Advice on Plumbers Forums.
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.