Kitchens – Remove stuck tailpipe from sink

It's called a tap connector (obvious now I've found it), and you should be able to unscrew it. You can see one here

It's the bottom nut you need to be turning. It's part of a joint that attaches the pipe to the tap.

Once you have this off you should be able to remove the tap as normal.

If it's stuck then something like WD40 or penetrating oil might help. Failing that carefully heating the nut with a heat gun might expand the various parts enough to loosen them. Take precautions to avoid getting direct heat onto the bath (it appears to be acrylic).

A more drastic approach would be to take a hacksaw to the thread to separate it that way.

While I hate plumbing, I do quite a lot for close friends and family. I don't know exactly what you are doing, but I know a couple of things, and I hope these will help. When you loosen the gland nut closest to the wall, you probably need to turn it clockwise. Normally when you loosen something, you usually go counter-clockwise. In this case the nut stays on the pipe, not the valve. Always you a back wrench like you were talking about to keep from twisting the pipe.

Here's a You Tube video that shows it. I hope that this helps and is not exactly what you have done before. Did I say I hate plumbing?