The thing with the specialist cleaners like the Nikwax and Grangers stuff is that they’re not particularly good at cleaning. Normal detergents – Persil, Aerial etc – leave behind optical brighteners and wetting agents that are bad for water-repellent finishes. But, what you can do is first wash the jacket in, say, Persil to get it properly clean, then re-wash it using something like the Nikwax cleaner. Double rinse it, then apply the reproofing product according to manufacturer’s instructions to restore the surface water repellency.
It might fix it. Might not. Depends on whether the waterproof part of the fabric – coating or membrane – has deteriorated or holed, but it won’t do any harm.
The heat treatment works by reactivating the original factory DWR treatment, mobilising polymer molecule chains or something like that, but don’t iron reflective / plastic zips / badges etc or you might kill them. Most aftermarket reproofers don’t need heat treatment these days.
Whatever Endura is selling will be rebadged product from someone like Grangers, Storm or Nikwax I’d have thought.