If you are cruising at a steady 70mph on a motorway and the TT has connection to at least 5 satelites (highly likely) thn it should be accurate to +/-1mph. If under these conditions your speedo is showing c63mph, then that is illegal !!
It is astounding that your local dealer says they cannot help. Most cars can have the tyre diameter reprogrammed via the OBDII port. Insist that they contact Citroen for the necessary help and advice, or do so yourself. In the first instance, they should be able to tell you what the correct tyre size is for your car. Assuming you have the correct tyres, then th eelectronics will need reprogramming.
You could try putting your registration into the Blackcircles tyre chooser and see what size it gives for your car. Does this match what's fitted? Not 100% conclusive, but a good indicator.
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