Alkathene might fracture.
I once had a clamp for compressing lead that consisted of 2 metal bars and two nuts & bolts that would squash the pipe closed but i never tried it on alkathene..... it might work :?:
they are the same size but sometimes are a bit tight to get on.
leave the valve all together and tap it on then undo the nut and check it's gone fully on.
Must be expansion then.
Either pipework or radiator and could be on the rad bracket.
Did you use the small plastic inserts on the bracket that stops the metal on metal expansion noise ?
A photo would be good.
Or get a different RGi out to issue a safety certificate and see what it says.
If there is a problem you could claim the cost back off the first RGi
The flue should be 2m away if it's below a rooflight or 600 above.