Normally compression type joints do not need any materials to seal them if they are done poperly. Check with your local Plumbers on how they do the joints in England where you are – every Country has different ideas and systems – they do not always follow the same.
Selgas Services Ltd Craftsman Gasfitters, Plumbers, Electrical Service Technicians
Peter is absolutely correct when he says that compression fittings (and this applies to flare fittings and machined unions as well) do not require any sort of pipe “dope.” These fittings depend upon clean and well-formed mating surfaces only. The only plumbing fittings that would benefit from pipe “dope” or Teflon tape are threaded fittings. As regards these threaded fittings, I use both Rectorseal #5 and good (mil-spec) Teflon tape with very good results.
In the “old days” some plumbers (like my grandfather and they guy I apprenticed with) used worked “plumber’s thread” and pipe dope into the male threads of the pipe joint, but no one does that anymore. I still have a couple of rolls of this special thread I inherited over 25 years ago, but I’ve never used it.