Fixing a connector between installed pipes!

[Angelheart]

Long story short, when my extension was sorted out, the plumber did not put an isolator on the hot supply. That loop feeds my en-suite and kitchen. When my heating was sorted, that was connected but there was a leak so my heating guy took off the connector. 22mm copper to 15mm poly. 
 

Now I want to add in the isolator and reduce. I have the parts now to do this but the previous connector and reducer is smaller than my isolator and reducer. Therefore I will have to trim one or other of the pipes back. This is a bit tricky as it’s all installed there and I’m only used to dealing with connecting things as I go and having free pipes. So my questions are, is there an easy way to gauge the correct gap I need to leave between the pipes and also, what would be the best method of connecting this all? 
 

Re my pics: Spigot reducer thing is not pushed into the isolator as I thought it may be better to install in stages as that is an easier one to push in last than the actual pipe. 
 

Pic of what I’m dealing with, 22mm copper from below, 15mm poly up top. 
 

Thanks in advance for any advice you can offer. 

[Nickfromwales]

Will the 15mm pipe push up into the ceiling void at all? You need around 40mm of movement to get these coupled.

 

There are 2 types of reducer. The one you have is a spigot reducer which goes into a fitting, and the other is a socket reducer which has a pipe inserted into it.

LINK

 

would that be easier than the fitting you have chosen, given you need to gain some length? You'd just need a short off-cut of 22mm copper to connect your isolator to the fitting I linked, and that should be job done.