Almost certainly the switch, or possibly the multifunction switch. If it is the switch, then flashing high beam should work. 
Unlike most modern vehicles, the 110/Defender does not have a headlight relay, so the entire lighting current travels through the switch (and the multifunction switch). The contacts will carry this current, but not with much margin. Any use of above standard wattage bulbs will result in early switch failure, and expect it to fail eventually, even if standard wattage bulbs are used - they are not very good switches. 
Depending on the level of damage to the switch, it may be repairable if damage is limited to burnt contact - simply clean them. But often the contact has heated up, and melted the plastic support, and in this case repair is unlikely to be possible.
An upgrade to fit headlight relays is recommended. This can be done by buying parts off the shelf, or one of the advertisers here (Traxide) sells easily fitted kits.
A search on this site will find a number of threads on the subject.
Hope this helps, 
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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