Never clean a helmet, dusting is one thing or wiping down with a damp cloth from time to time. Like many collectables, cleaning actually decreases value.
After looking at the placement of the rivets, your shell is a WWII made Mk III shell with a WWII era liner (The Mk IV had the rivets placed much lower). The repaint could be a WWII repaint or later, you never know, remember D-day happened June 44, the war ended in May 45. Lots of battles in-between where it might have been repainted.
The link I sent is straight to their British area of the site, it's free to join and you'll get some good information and might even get a better current price range.