Ok update.
I tried a replacemant GPS antenna, but as I expected, it did not fix the issue, and the nav and location error(s) were exactly the same as before.
I note that despite all the info I have, including the Mercedes workshop manual and parts system, the GPS antenna on my car (late 2015 C Class W205 amg line premium plus), the antenna is in the right front mirror housing and not the left hand side!
So, I am going to have to resort to booking the car into the Mercedes dealer for diagnostic and hopefully/possibly a system re-flash/update, may be required to fix the issue.
I will update again as and when I have more info.
UPDATE! Fault fixed. Note this is related to NTG 5.2. as fitted to C Class W205
So, as briefly as possible...I decided to get my own xentry and vediamo diagnostic system. (mercedes dealer wanted £180 just to do a diagnostic check, with no promise of finding the problem, and then quote for any repairs to fix).
The Laptop with openport vdi interface and xentry, vediamo and Monaco etc cost £259 on eBay, which is obviously very useful to have regardless.
Diagnosing, with xentry showed NO faults at all with the headunit or GPS antennas etc??
Further, research into the issue, I found that the NTG5 head unit has a uBlox chip which handles all of the GPS/Sat Nav data processing. This chip also has a onboard gyro sensor.
From the way my fault was presenting itself, I had already determined that the Sat nav was receiving GPS data from the GPS antenna OK, (confirmed by Xentry), as it appeared that the system was attempting to determine and display the correct vehicle position, (as it was always fairly close to the actual location, but would struggle to stay accurate, especially when moving, which meant it would wander off route and result in "route/road not mapped" message and the location shown not accurate (usually off by upto 50 meteres or so. The software was apparently trying to work, as it would "snap"the vehicle location to the nearest road.
However, the compass and direction of travel on the map display was consistantly incorrect. the compass would show a vehicle direction heading of 90 degrees or more out from true heading.
The sat nav map display would show correctly show the direction of travel arrow pointing upward, (as is correct when set to "direction of travel up" as opposed to north up) but this resulted in the map often scroling horizontally (with the display showing the vehicle travelling sideways and not in the direction that the arrow was pointing. Normally, the map display should rotate so that the road ahead is vertical and in line with the direction arrow, and the maps thus should scroll vertically.
As said the actual location was close to being correct, but the system appeared to be getting conflicting information from the live actual gps location and the compass direction of travel.
From all of the above I decided that the electronic gyro must be faulty, and this is not repairable. (without replacing the uBlox chip).
I subsequently purchased and installed a used NTG5 comand head unit and all is good.
Fitting the head unit is easy, but it does require the "Antitheft code" entering and I had to do some "variant coding" changes to match my vehicle (Using Vediamo). The Antitheft code is tied to the head unit serial number, and the code was pruchased from a well known online shopping site, costing £18.99.
Also, the map data is stored on the head unit hard drive, and this is tied to the vehicle VIN number. As I already had the latest V15 map data on a USB drive and pin number, I simply installed the map data again on this used head unit (after unlocking the antitheft), and it works perfectly again!
FYI. If you change your head unit, try to get one that has come from the same model vairant as your vehicle, as this will mean it has the correct vehicle model image displayed and the majority of settings will match your veicle.
The unit I purchased was from a C63 AMG (mine is a C250 AMG Line premium Plus) so, some settings were significantly different, particularly the vehcile model show was a C63 petrol and the "Dynamic drive" settings did not function correctly until I changed the vehicle from AMG to sport model.
In AMG varient model mode, I did have the AMG engine/performace data screen with the dials etc. (which was nice) but there was no data input so didn't dispaly any values, so useless anyway.
Apologies for the lengthy post, but hopefully it might help anyone with the same issue, as I found very little info online, except the common issue of faulty GPS antenna, which in my case was definately not the cause.
Finally, as a side note, I am not sure if the existing maps data will work if you swap the hard disc from the existing unit, (the map data and also the vehicle user manual is stored on the hard disc). And, I do know that the GPS chip is located on a "daughter board" in the head unit which plugs into the main board, again I'm not sure if swapping the daugter board would work without it activating the "AntiTheft" protectection.??