Rebuild of the 1990 TT jap auto going well, engine back in and running.  Worked through all fault codes and just code 26 left, all voltage checks ok, reading +-2v as pressure changes , replaced the sensor anyway but code remains, yes I'm resetting the codes each time.

I'm starting to wonder if it does relate to the boost sensor as that doesn't appear to be connected to the eccs just the gauge. Does it actually refer to the wastegate solenoid circuit?

Any help/ thoughts appreciated. 

Do you have the original 8-bit ECU fitted? The later 16-bit ECU's fitted to '95+cars do have a seperate boost sensor and signal input to the ECU

so will throw a 26 if it is faulty or missing which you will get if the ECU has been changed to a later type.

Brilliant AndrewG. I had assumed it was original with a Zedworld remap but it has a note saying 16bit improved, so guess it was replaced. 

Problem solved thank you