How To Use 15mm To 10 mm Microbore Pipe Reducer in Radiator Valve? | elegant-radiators.co.uk 

No, it is for 15mm pipe. You need reducers if you have microbor pipes that8 or 10mm

Yes, you definitely do need a liner/insert if it's poly pipe on a compression fitting

JG (John Guest) recommend NOT using reducer on plastic speedfit pipe (e.g layflat or PEX) or on Compression fittings with a short tube stop depth (like valves) because they have a hard olive which can easily damage the plastic. And inserts always when connecting plastic to ANY fitting regardless of size to retain integrity. I used this video to connect new towel rails using layflat and reducers and learned the hard way that what is shown is a bodge! See page 16 of JG guide www.johnguest.com/sites/jg/files/2022-01/RWC11339_JG-Speedfit-Technical-Specs-Guide_v11.pdf

@@elegantradiators Do you even know what an insert is?

Just learnt that the hard way! Will drain down again tomorrow and refit! Electrics is so much easier! Lol

Managed to get the insert round the right way at the second attempt. It was a video not live feed. Could have done it right second time.