Okay, so following a comment from one of the 4Fuel team, I have conducted a second test with their gel fuel. I allowed more room for the fuel to burn by setting the pot higher on the stand. The flame is just visible.
The result was that the water actually boiled, in 11 minutes. This places the gel somewhere between solid fuel and liquid fuel.
But I had to add to the gel a couple of times (very carefully) in the time taken else it would have run out and no boil would have been achieved. So to boil half a litre of water I would have to approximately half fill the stove with gel. The amount of gel used did maintain a rolling boil for just over two minutes. So enough to make safe drinking water.
So yes it boils, but it uses quite a large amount of gel to do so, and still leaves an awful mess in my stove afterwards.
For me, I'll stick to the 4Fuel spirit as it suites my needs. The gel will come in handy for starting barbeques etc. Your needs may differ. I'm not slamming the gel, it does the job, but it isn't as useful to me as the liquid bio-ethanol.