Could be the heater control unit.
Contact Julius from this eBay link. He breaks c5s. I've had bits from him. Very helpful.
https://www.ebay.co.uk/itm/163720125396?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=J_mXsPy2Qdi&sssrc=2349624&ssuid=IE07kO4STZy&var=&widget_ver=artemis&media=COPY