Thanks again to each for their suggestions. I've just been out in the cold for an extended period - the tank is now working absolutely perfectly! The ambient temperature in my flat is always high (too much technology), could it be a simple matter of the tank being too hot at rest?! Would it be folly to try popping it in the fridge for half an hour if it plays up again?
You've just discovered the secret of the Nautilus Mini and most other bottom coil tanks, and it's not a coincidence it works fine in cool weather. I've had my Nautilus Mini's for a year, and its a love/hate relationship - love them in cooler weather with the vertical cotton coils for my mouth to lung style, hate the gurgling when it's hot. Right now with this cooler weather, I love mine, its performing just fine at only 8 Watts, even with 70pg/30vg (a thinner juice, even more likely to gurgle than a higher VG juice). Great flavor, great performance.
The reasons for gurgling of a bottom coil tank like the Mini are twofold:
1. If you go from a warm to a cooler environment, the air in the tank expands, which serves to push more juice into the wick, over-saturating it, the result is gurgling.. In the summer here in NC its hot, and when I go outside, I immediately get gurgling, too quickly for the cause to be the liquid warming up (and thus thinning out).
2. The viscosity of PG and VG significantly decreases in warmer/hot weather, making it much thinner and thus flooding the coil wick, again resulting in gurgling. This occurs even if you aren't changing temperatures as in 1 above. If I am outside, and the temperature is above 80 deg F or so (26 - 27 deg C), I get gurgling. The higher the ambient temperature, the more gurgling occurs. How hot IS it in your flat, I wouldn't think it would be so hot as to cause problems, but it sounds like it is....
I don't list power as a cause, since if you are operating coils in the range they are designed for, the thing should work. In warm weather, it just doesn't, so I call cranking up the power a work-around, not a cause. The causes are over-saturation and flooding of the wick due to the causes listed above.
I'm a tootle puffer, using 18 to 24 mg nic, 70pg/30vg, for good throat hit and mouth to lung, like i said, I prefer around 8 W, maybe 9 depending on the juice (I like Naturally extracted tobacco juice, but also desserts as well). What other people do to minimize gurgling is crank the power up, in doing this they are essentially vaping enough liquid fast enough to burn off the juice and pull the liquid down some which will help establish vacuum in the tank, which in turn helps prevent over- saturating the coil. But I just don't like a hot vape.
First things you should consider - Make sure you have an authentic nautilus mini, and authentic nautilus coils, check the code on your packages on the website as directed for your tank and your coils. And as djsvaopur wrote, are you sure your battery is good, and charged? Even in the hot weather, I got vapor even when gurgling, just not as much without cranking up power. And assuming that's all good, keep reading...
Things I do to minimize gurgling in a warm environment:
1. Make sure my coil is fresh and not overused -I replace my coils every 3 or 4 days or so when they start tasting a little harsher than normal, NETs are tough on coils and gunk them up quickly).
2. Don't overprime when putting in a new coil. A couple of drops down the center of the vertical cotton coil, and a drop or two on each of the wick holes. After putting in your juice, take a few unpowered (dry) draws to pull the juice into the wick, let it sit 5 or 10 minutes... let the juice in the tank do most of the wicking, that way the level drops and a small vacuum is established in the tank, helping to reduce over-saturation of the wick.
3. When I refill, I take my tank apart, rinse out the glass with tube under the hot stream of water in the faucet, dry the inside of the tube (see the juice there?) with a twisted end of a paper towel or napkin, wipe out the inside of the coil gently with a twisted end of a paper towel or napkin, just as Navigator2011 suggested above. Rinse your drip tip out too, its coated with juice.
4. In hot conditions, for storing at night, take the tank off your battery, remove the drip tip, and sit it upside down (drip tip side down). I only do this in summer. usually it stays on my istick 30W. (I have two actually).
5. Vaping - Pushbutton has it right above, its all a balance - try vaping on the larger air hole, and if you need to take the power up, try that, as long as you are comfortable with the hotter vape. I myself don't like to go any higher than 10 to 12 W, and my preferred power is 8W as noted above. When its 90 deg outside, I can never get a balance that is satisfactorily to me. Subohm folks don't have as much issue, since they vape at such high power the juice doesn't have time to saturate, haha.
Bottom line is this is winter. Your Nautilus should be working beautifully in this weather, so something is wrong with it, or your battery, or other. I like your idea of putting your tank (not the battery haha) in the fridge for 20 minutes or so. Try it. Let us know. I might try that for summer.
Good luck!
because I know that it (sometimes) gurgles because of it? 
