The instructions to fit a clear bumper protector to my Superb estate specify the use of Terostat 8550 cleaner / degreaser for the paintwork. I cannot find any available for sale on Amazon etc so is there a more commonly available product that does the same job and could be used instead.

IPA (Isopropyl Alcohol) is ideal

It looks to be a Henkel/Loctite/Teroson product that isn't sold in the UK.  The french safety data sheet here indicates  that it is isopropanol based product.  I think it's just a glass/paintwork cleaner and degreaser and any isopropanol cleaner will do - freely available in aerosol and bottled packs.