Help changing tap

[SDMFer]

First things first - I didn’t install this, it was like this when I bought the property! I need to replace a kitchen sink tap but am struggling to undo the screw fittings between either the copper pipes and the tap or between the copper pipe and the fitting to the plastic pipe - attempting to unscrew either causes the plastic piping to simply rotate. I also tried to remove the plastic pipe from the 90deg fitting at the bottom but little success. Am I simply doing it wrong, is there a trick to unscrewing the fittings or is it rubbish and needs redoing from a pro plumber? Photo attached (edit: attached link to the image as I can’t seem to insert it)
https://1drv.ms/u/s!AvZMI0--dmE6hIxEc2YQU793iMkqdQ

 

[fobose]

I can't see your picture Plastic fittings have a collar on them that you have to push back against the fitting whilst pulling on the pipe. A good video of this here (skip in to about 1m 40s): https://www.youtube.com/watch?v=ab0bapj_f7M

P.S Make sure you have isolated your hot/cold water supplies if not done so already! There will be some water left in the pipe so have a towel and a small pot handy.

 

[FlyingFish]

I take it the copper pipes at the top of the picture are the two tails to the tap (hot and cold). Normally with the hot and cold isolated (Looks like you have iso valves right at the back) you just need two suitable spanners. One to hold the brass body and the other to undo the top nut.
The plastic elbows below that rotate (are not John Guest) need a special key to release them (HepKey by the looks)

You will spill some water so have a bowl, and you may need a set of monobloc spanners (Rothenberger £10, screwfix) to undo the tap and fit the new one.

Have fun