Thanks for the warning Wishiwascorgi
I'll stear clear of them for my central heating just use them on other work to be done, if necessary. Or back to compresion fittings.
I've done solder fittings but the last few I did had little pin holes in the solder ring and they ended up a compression joints. Because of the water in the pipe the re-soldering wouldn't take. I tried a dodge a plumber told me that was to ram bread [without the butter] down the tube to soak up and stop the water rising while soldering it didn't work, probably not enough bread as we'd run out of dough ha! ha! ...........sad ennit!.