I've got a 2004 Mark 5 golf and my rear screen heater is not working. The light comes on when I switch it on and the side mirrors de-mist and the switch turns off as normal after some time. Any ideas?

Clean the defogger wire connectors with abrasive cloth and apply a thin coating of dielectric grease. This link should help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-11.html#post2138426