finning technique

split fins only work with a high cadence, low amplitude kick. As for fibre fins, I'm currently picking between a pair of Breiers or a modified pair of leaderfin spearfishing fins. Dunno how well the second will go and I've heard about durability problems with the first.

yeah forget split fins mate. ok for training, ok for muck around games, but pretty rubbish in real games.

cheap options which aren't too horrific might be mares avanti tre, maybe technisub stratos (bit stiffer), possible mares excel (a bit light).... or something similar to those.

fibre fins are obviously good but the briers and leaders both have issues with snapping, plus the briers and others like them also have to be constantly glued back together when rails have been added... probably nemos seem to be the most durable fibre fins, i havent heard of a pair of them breaking yet either (though due to the material im sure its possible)

nemos need some good socks or latex booties to go with them.

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it's more polite on the grating than on the subs bench

I currently swim with the first version of the Breiers and Liam is right. They have to be reglued on a regular bases. Especially now the sides have been added.

During the Europeans, some players had a new version of the Breiers, with a closed foot pocket of Beuchat or Immersion. The two main differences I saw, was that the sides were much better fixed and the fins became a bit stiffer. The regular Breiers are a bit too flexible in my opinion.
The pocket with the Immersion also had a velcro on the top to strap the fin better to the foot. Interesting concept if you ask me.

Problem though is the price. They cost around 190 or 200 euros, which puts them in the higher price range of fiber fins.

" Different strokes for different folks"
start with Froude number...

By wide kick do you mean deep kick? And by narrow do you mean shallow? In most circumstances the kick is going up and down so it should be described relative to depth rather than a horizontal reference. You know, for the beginners out there that are trying to figure this thing out. I imagine someone working a sidestroke kick trying to get reallllly wide.

As far as technique, years of swimming will give you a good base of proper technique but it might be too late for that. So if you can get help as Zoe pointed out, do it. If not, spend time in the water experimenting with your kicks. Work everything from slow deep kicks to slow shallow kicks and fast shallow kicks to fast deep kicks. And keep mixing it up a bit. Here's something to get you started...

The basic flutter kick:
A good drill to get the right feel for a kick is to kick so that it feels like you aren't bending your knee at all. That forces you to start your kick up in your hips. Although it's not identical, I like to think of the movement as that of a balerina doing those fancy little jumps while their feet flutter. Everything is tight so you get a real good feel on both the downkick and the upkick. Despite that you will think you aren't bending your knee, you will be. But don't worry about that unless you feel like you're bending it. Do this while kicking only 8-12 inches (20-30cm). Once you feel good about that (maybe 10-20 very focused lengths later) extend it to 18 inches (45cm). Later, add the feel of a slight bend in your knee as you start the downkick.

Don't try these with big fins on. Either go bare foot or with zoomers or with the wimpiest fins you can find. This is a drill, not a competition.

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Just in case you weren't sure, it's official -- CMAS sucks.
Oh yeah, and now they're broke.