Bosch PHG 500-2 heat gun, prior to cleaning the internals, and a spot of fault finding.

Motor brushes were worn out, and aren't easily replaceable. You can buy a complete new motor and fan assembly. But your £21 for that, would be better put towards the £27 for a complete new heat gun - delivered - with warranty. Such advice was passed on to the Painters & Decorators. They'd had it for about nine years, so it had done well!