Run a sharp knife between the wall and base first. The white sealer is acting as a glue. Then twist and pull.

I have done this with scissors - the glue appears to be broken. The plastic bit spins freely around the pipe, but will not budge forwards or backwards

Also when I pull the pipe, the whole thing comes with it, and seems to pull at the pipe behind the wall

Try pulling the fitting while turning it counterclockwise... keep doing it and see if the fitting starts moving out.

Can try using something(hammer, screwdriver) to pry it off, but will need about 1/2 inch wood/plywood to protect the tiles from cracking. Looks like you have a new replacement handy, so might just use a saw and cut it off.

I’ve done a lot of anti-clockwise spinning/pulling, and nothing

Thats a tricky one: Power tools can easily damage the pipe and open flames are out of the question. I guess the best way to do this is to slowly eat that thing away with some strong diagonal pliers. Perhaps try to soften it with the heat of an hair drier

I don’t own any tools other than a screwdriver haha, but can get some if needed. I don’t really know what I’m doing, I just broke the shower and need to fix it, and am trying to avoid calling out a plumber

I think the easiest thing might be to get the matching shower arm

The existing one was extremely outdated and poor quality, it’s just chrome plated plastic. We have bought a nice high quality one with a rain shower attachment to use, so we would really like to be able to use that if at all possible. I can’t believe a shower arm would use a fitting impossible to get off? Surely they know people would want to change this at some point?

This is the ugly existing one: showerspares.com/search/…

A small hand saw might be your best bet and remove the plastic housing in pieces. With plastic you do not need to be picky on type of saw.

I think I will try that! Could a hand saw also go through the copper pipe, if I am not careful?

Do you have a Dremel tool? a cutoff disc would probably do the trick. Be careful about pulling on it if you don't have access to the plumbing behind the wall. And be extra cautious about using a screw driver between the tile and that fitting, because tile is pretty easy to crack and not easy to fix. And yes, a hand saw can easily damage the copper pipe (but so can a dremel... so no matter what, be careful and go slow)

No I don’t, the extent of my toolbox is a borrowed screwdriver haha. But I can buy a saw if that’s the easiest thing

Based on your DIY inexperience I would recommend having a plumber do this; even a reputable handy-man should be able to tackle this for you. Paying $150 for their time is considerably cheaper than you breaking something. If it were me then I would get a sharp wood chisel and carefully hammer it into the plastic parallel with the pipe to split the plastic open.

If I could pull the pipe away from the wall a bit, would a tool like this be able to get inside the plastic thing and shove the teeth away? Or is there not enough room for it to get inside? screwfix.com/p/hep2o-push-fit-demounting-tool-set-4-pcs/1208‌​j

Literally done the stupidest thing ever, to see if the new grippy thing would come off any easier (to see if the other one was rusted on) I’ve stuck that one on on top. Obviously it didn’t come off any easier. Going to have to try and saw that new one off and just use the old one. Couldn’t make it up haha. Thanks for all your help, I’ll be leaving the old one on there, assuming I can get the new one off

This is going to be hard to describe, but grab the fitting in your fist and slowly rotate your fist around the pipe while pressing the fitting against the side of the pipe. You are applying the side pressure at the base where the teeth are located. What you are trying to do is flatten the teeth so the teeth are no longer sticking out. A few rotation of your fist with pressure along the side of the pipe should be enough to flatten those teeth. You should be able to do this with the old and new plastic fitting. My other suggestion would be just use the old one, which you thought of.

I will just use the old one, but the new one I have shoved on in front of it! Hahah. So I need to remove the new one to free up the old one

Heat it up with a hot air gun (or a naked flame if you youre confident you'll recognize when to stop i.e. before you set it on fire - just lick it with the flame, don't torch it continuously in one spot). Chances are the plastic will be thermosoftening and you can just pull it off(with gloves) as if it were floppy rubber. It might leave the metal teeth behind but they will be a lot easier to remove (pliers) with the rest of the plastic out the way

The problem with using the old one was just that there was meant to be a spacer between the wall and the new one, so I was trying to remove it to add that p

You have another very similar question. diy.stackexchange.com/questions/229405/changing-shower-arm