Peter, are they standard or aftermarket? Float gauges in my tanks are totally useless, and I am thinking of replacing them with something better - any recommendations?
They were standard on the GT, and may have been fitted to pre-GT serial numbers too (I have no idea).
During inspections etc I have looked for the transducers. I cannot see far enough into the wing (through the filler hole) to see what is inside the fuel tank but I have seen the "dry" side of it, and it seems to just go into a hole which - at a vague memory - is something like 50mm diameter so could be retrofitted easily if you can get access to the most inboard wing rib i.e. the most inboard end of the fuel tank.
The first page in this PDF shows the standard (old) fuel metering system whereby some potentiometers (with floats attached to rotating arms) are used. That system is used in nearly all of light GA and is basically crap. The other 3 pages show the capacitive system which is very good but a lot more expensive. As with most of Socata's wiring diagrams, I have no easy way of telling which of the three applies to my late-GT aircraft but you can see the basic idea.
This PDF shows the actual parts. At the beginning it shows the old system with the potentiometers and later on it shows the capacitive transducers. I don't know who the manufacturer is; Socata allocate their own part numbers to most things so you can't find out, but it may be stamped on the components themselves. They are most likely off the shelf French parts.
It looks like Socata use two sensors in each wing tank, so that aircraft pitch is compensated for, but not roll if the aircraft is out of balance (yawing). The Cessna manual uses a lot more sensors - an impressive implementation.
We usually land with around 1200lbs, despite the best efforts of London to have us burn more by descending us to FL260 20 before Biggin; far too low too soon.
They do that to you as well - I thought it was just me being in a little plane. I tend to plead now to be kept high, not sure it makes any real difference. I tend to get FL200 20 before LAM - still a long way from Oxford.
I fly a CJ2+ and the poster earlier has been more comprehensive than I was going to be by posting a link to the manual.
I believe the CJ4 uses basically the same system.
We find the accuracy to be as Peter observes.
Fuel is a critical part of jet ops, it is quite normal to take off say from Annecy with around 3000lbs of fuel on board, and we will have the FMS telling us we do not have enough fuel to get back and giving us warnings to "Check fuel at destination". We usually land with around 1200lbs, despite the best efforts of London to have us burn more by descending us to FL260 20 before Biggin; far too low too soon.
There is an interesting STC fuel sender being developed by www dot ciescorp dot com. They are currently available for the Cirrus. The Mirage people are quite excited as the senders in the PA-46 (ex-Meridian) are notoriously bad.
I have "expensive" capacitive fuel gauges in the TB20GT
Peter, are they standard or aftermarket? Float gauges in my tanks are totally useless, and I am thinking of replacing them with something better - any recommendations?
In the Meridian there are three tanks on each side - a main (outboard tank) holding 64 USG, an inner tank holding 12 USG and a header tank holding 8.8 USG. There are three sensors in the outboard and one each in the two inboard tanks. So 5 per side. They are wired in parallel so the total capacitance is summed and used to compute the fuel load by the DAU.
More sensors and averaging?
The rather coarse tape indicators of our Citations allow reading the fuel quantity within +/- 50lb (of a total of around 5000lb) which is an accuracy of one percent. Good enough!
The resolution is not a problem to achieve. Resolution is cheap 
It is the absolute accuracy which is impressive.
I have "expensive" capacitive fuel gauges in the TB20GT and the indicators are accurate to about the thickness of the needle i.e. a few % of full scale deflection.
But the CJ4 accuracy was of the order of 0.5%.
What sensors do they use?
As well as cost, I assume the reason that these sensors are not used so much in GA is that turbine engines (at least mine) are far more critical wrt to fuelling vs range. ie for a given fuel load there is very little you can do to affect range other than fly at the optimum altitude. You can slow down - that increases time aloft but does very little to improve your range. As a result you have to pay very careful attention to fuel - more so than in my piston time.
PS - and what next mine gives it to me in 10lb increments on an 1150lb load. So pretty similar levels of accuracy.
