Fuel

I don't stint myself on hot drinks and meals in cold countries. 

Alcohol fuel consumption

• Stage 1: Just under 1 litre
• Stage 2: 2 litres, of which the second was bought at the supermarket at Lake Myvatn
• Stage 3: I picked up the fourth litre in the second resupply box. In fact I only needed one day's worth of this

Gas
If you are using gas then these canisters seemed very expensive but are available in Reykjavik. I did not see them in the supermarket in Lake Myvatn. I think they were available at Landlammalaugar, but it would be safest to include them in your resupply box.

One tip 
Fuel is expensive in Iceland ($12 for a litre of denatured alcohol). 
The best stuff was the Bio Ethanol I got in Reykjavik - try the main street hardware store. It was clear and burned cleanly. 
The pink stuff burned a bit too fast and was a bit smoky. However adding a little water to the fuel largely solves this problem. 
This is apparently a well-known trick, but I had to discover it for myself on this trip.