It can be either the combination switch or the main light switch. The headlights do not operate through a relay, and the switches are only just up to the job. Any increase in wattage of headlights without fitting relays means an imminent failure of one or both switches.
The relays fitted with your extra lights almost certainly did not add relays for the original headlights. 
I would suggest that you find which switch is at fault, replace it, and add headlight relays as suggested.
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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