The tank was nearly full when I left for a week, when I came back it was almost at reserve. The temperature was nearly 40 celsius during the week but I doubt that all that fuel evaporated through the breater tube. There isn't a strong smell of petrol under the tank either. Where am I losing all that fuel?

its prob in your sump , leaked through the carbs

check you oil level , bet its sky high or a puddle in the garage

what he said ^^^^^

Hopefully someone stole some... that would be an easier fix

The oil level is high when I check with a cold engine but it is within limits if I run it a bit before measuring. I think that's normal.
When I disconnect the fuel hose and switch the tap to reserve there is no leakage. I can't explain this one...
I got a lot of oil or petrol on the plug threads: https://plus.google.com/photos/11203432 ... gImq9J2gLg

First of all let me say that I know cars way better than bikes but it looks to me like your bike has been running rich and the oil on the rings was probably sitting outside the head when you removed the plugs (unlikely that it came from inside). When you check the dipstick check the bottom to see if there is clear liquid (oil floats on water probably the same on petrol) the other day my bike had a pool of fuel (I think) under it which seemed to come out of an overflow (I haven't checked how the fuel works yet) possible this may have happened to u and the fuel evaporated.

Why would it run rich, the float height is checked and the fuel trap too, so odd the jetting?

I am not sure it is running rich, just visually the tops of the plugs look too black, they should be a dark biscuit colour (could also be the result riding the bike gently as well) and yes that would be from jetting. The only way to be certain would be to run it on a dyno and check how it is running... without being funny are you sure someone didnt syphon some fuel out of your tank / are you sure it was full when you left?

No syphoning I'm sure.
The only other explanation is that the tap is leaking intermittently plus there is a leak somewhereelse down the line. I'll disassemble the fuel tap.

How can you test for fuel in engine oil?

You should be able to smell it on the dipstick, maybe put a small amount ( like 50 mils) in a container (and at a safe distance) see if you can light it. But from a logic perspective, I dont think petrol in the oil is your problem, consider that an NC30 takes about 2.5 litres of oil (obviously that is the required oil level and not the crank case / engines total possible capacity) add probably about 10 litres of "lost" petrol (right?) and the block should be full of / overflowing with petrol. In fact the fuel would have most likely seeped above the piston tops as well and this would mean the it would stop dead if you tried to crank the motor. I think that maybe there is a seal (or something similar) in the fuel tap that when expanded (maybe at the 40 degrees in your garage) is causing the tap to leak. Maybe leave some newspaper on the floor under the bike and see if it is crinkled (fuel onto it and then evaporated) after a couple of days?