Ok I've finished my unscientific experimentation, Greenheat vs Meths, let the battle commence!
The Greenheat 'fuel cell' is a tidy little thing, I'd feel fairly safe throwing an unopened one in the bottom of a pannier.
Eww...Icky
The Greenheat 'fuel cells' come with a springy ring thing that allows it to sit in the Trangia burner hole. Helpful marks on the tin show where to position the ring so as to bring the flame higher for the frying pan or lower for the pans.
Here it is sitting neatly in the Trangia. The gooey green slime lights very easily, at the first flick of my lighter, though you can't really see the flame here.
A full kettle (25 series kettle, filled to just below the spout, more than two large mugs of water) placed straight away onto the flame.
Some 18 minutes later the kettle was boiling, not exactly speedy as has been suggested. Ok it's more water than I'd usually boil, but you certainly couldn't call it a 'quick cuppa'.
The greenheat is extinguished with a quick blow, something you can't really do with the meths burner, and very quickly it is cool enough to replace the plastic lid, which fits tightly, but I wouldn't be so keen to throw it back into a pannier now. The green stuff looks really quite messy, I'd hate it to empty itself over my clothes. I'd probably tie it securely in a plastic bag before packing it up. As a plus the underside of the kettle is completely unsooted, which is nice. Very little of the fuel appears to have been burned.
Now the same procedure, same kettle (cooled) on the meths burner, unwatered meths.
It still seems to take an age, watched kettle and all that, but after 11 minutes the water is boiling. So as suggested, considerably quicker.
So in conclusion, Meths certainly wins as far as flame temperature, however honourable mention has to go to Greenheat for it's neat container, spill resistance, lack of soot and, apparently, more environmentally friendliness. I've no idea how it rates against meths for weight / heat output. Maybe speed of boil is less important to you than the rolling weight of your packed touring bicycle, in which case I guess more experimentation would be needed.
As a little bonus experiment, I let the meths burner cool and then boiled the same amount of water in the large pan, with frying pan lid. As I'd always suspected, this was a faster way of boiling the same amount of water than in the kettle, taking just 7 minutes.
Still, there's something comforting about having a kettle, and it's easier to pour.
