Zaavi sent me a marketing email in February and it worked. They were offering a Hasbro Darth Vader Helmet complete with sound effects for £99, cheaper than other sites like Amazon.
It comes in more than piece and after fitting a collar around you neck, the helmet fits over the top. I have a large neck and it fitted find for me and is adjustable so should fit a wide range of people. The eye pieces are very dark from the outside but you can see through them from the inside. This should make an excellent costume piece for Halloween. Not to be confused with cheap costume helmets.
The sound effects are good and there is a small button on the neck collar for making the breathing noise. It also makes a noise when lifting off the helmet.
I think I will buy the rest of a Darth Vader costume, a Master Replica light sabre, use this helmet and make some electronics for the chest piece to make a realistic looking costume in time for Halloween (if we are allowed out of lockdown by then?)