Hi Mick
I had a similar problem with a C11 which was solved with one of this type of device :
https://www.bigw.com.au/product/wilt...pener/p/578094
I now have a range of sizes of these, than have been useful for other stuck threads, such as T rings, camera filters etc.
If still stuck, you could try a plumbers rubber pipe strap wrench, or an oil filter wrench.... with increasing probability of paint loss/scratches
cheers