In the boat marine world a similar thing can happen .
To be precise ( if I have understood your post correctly ? ) your guage reads high with no alarms , codes or faults in the system detectable and the engine runs fine ?

You not said if they used a VAG com Bentley specced upto date code reader ? These are pretty good I,am lead to understand.

Your techs have chased down randomly replacing parts ***** nilly running up a bills without a diagnosis.
If it was the sensor the VAG com computer would say something like “ implausible signal “ which means the ECU does not recognise the outputted voltage , Or “ short to GND “ or what ever ?

But in the boat Volvo penta world ( computer diagnostics too with a vodia tool ) ,With so called false overheats , ie the guage only it the wiring harness that ages and although the sender sends out the correct signals it’s gets corrupted by increasing resistance of heated wire insulation so the guage gets the wrong sig and reads high ,Somehow the ECU,s and thus diagnostic devices don,t pick up on this .

changing sender / guage makes no difference except generating a bill !

In the boat world we carry infra red thermometers ( £10 from a electronic shop )
get one and run to temp and point it about inside the engine bay especially at the radiators to see what the exact temp is .
Test parts of the block too if it’s around 90:- normal then it’s not actually overheating it’s the instrument or the wiring to the instrument.
sensor , overheat alarm etc are all ok .

so you need to eliminate a true overheat with a IR thermometer.

Once you have a true temp reading the first thing to look at is the fan turn on switch thingy.Are the fans coming on at the correct temp .Without a IR thermometer you are just guessing really .

hi johnny thanks for your reply. I do apologize for the miscommunication, I’d like to state where i am located & what more Bentley i own, 2006 Bentley flying spur w12 black red interior with 50kish thousand miles with full service history, a clean motor,

I live in central London at the moment the weather is 40°f (5c)
however I purchased the Bentley beginning part of summer when temps exceeded 87°f /30°+Celsius

What i did was replaced the battery to appropriate from independent Bentley service, exhaust gas sensors, 2 coolent sensors, coolent flush with a vaccume,

the car would start overheat faster with the ac on in the summer, however when switching over to heat rather then cool air it would drop to its normal temp just before 90,

nowadays temps a lot fresher & heater needs to be on because it’s bitter outside it takes a while 45mins. or if i drive it in Manuel meaning stressing the engine & gearbox it starts to peeks above the 90 mark 100°c & the fans start blowing full power, it slowly but surly drops to its normal temp 90°.
It is reading no fault codes every fault code it had was sorted. i did point the IR Gun everywhere possible hose’s engine block radiators “ECT” and it ranged from 89° to 91°c everywhere witch i was told it is normal By a Bentley tech...

Sometimes I think the independent techs get left behind with updates for the management readers software.
I know that happens in Ferrari world where Ferrari closely guard the soft where so disadvantaging Indy’s .
How ever with VAG com I don,t know where they are with the Indy’s and trade and where they are going .
Just seems odd it’s does not throw a code . Or the Indy can,t delve deeper through a few pages .

Not entirely related I live quite a distance from an official B dealer and wanted a small part , it was the headlight washer relay .Thet would not come on the washers and I eventually correctly diagnosed a duff relay .
Mr Google did not work to source the part .
We run ( wife’s ) an Audi the centre is 7 miles away and it’s just had a biggie , new disks , tinning belt etc @60 K miles along with some 4WD trans axial lub change ,
Garage knows me .
So I waltzed in with the Bentley relay in hand to the parts counter .This was Nov 17 .
They guy was v helpful , knew all about the x reference part no,s the shared parts , showed me some A8 disks , callipers etc but and it’s a BIG but he said at the moment his VAG com system gave him access to whole groups areas inc Bentley etc but from April 18 they were going to install just a dealer modal specific soft where so for them Audi only .

Then he just wrote down the B part no and his system then identified the Audi part a quick £ 14 and I was off and it fitted and worked ,The system listed all the modals it fitted and I could see different part numbers for each marque .

I understand this functionality has gone from April 18 in the VAG com group parts dept with the latest software.

I think Bentley dealers charge in the U.K. a straight £200 ( vat inc ) for they say about 1 hr for a tech to do a full diagnostic.

I had that done when my UK Indy hit a dead end with the HVAC problem , fortunately the Indy was an ex employee and the dealer on good terms so the Indy was able , by invitation or ring the Franchise and the tech spilled the beans so to speak .
Franchise is very busy anyhow so happy to help the Indy in this case with older modals , and I was on a tight timeline anyhow .
They did offer me a fix @£240 / hr labour and more expensive parts as well .

So you are back to why no fault code ?
I suspect your Franchised dealer will find one ,
The Indy fixed the issue for 1/2 the quoted estimate from the Franchise.
Point is his system could not detect a fault but the Franchises did and I got a fix .