In reply to barrow_matt:
I tried using fuel blocks, a bent tuna fish can with some holes in it and a titanium mug at the RAB last year. It was terrible. It took about 25 minutes to boil a mug of water and all my food tasted like the fuel blocks.
I think that fuel blocks are probably light but I would carry an army "stove" basically a light metal tray with legs and holes in it so the blocks burn efficiently.
I use lightweight gas stove normally, pocket rocket or similar, you could get away with a half full small canister if you aren't going to brew up much.
If you email me I will send you my kit list which details everything so you can see where you are going wrong/right.
15L bag is pushing it though, I think unless you have a very small tent and light sleeping bag you will struggle.