01 Saxo mk2, ECU warning light problem - lesz42

I have a Saxo vtr 2001 (Y reg) mk2, 80k on the clock, in hte past few
months done a service of , sparks, oil, gear box oil, coolant. A few days ago, my engine warning light came on, so I checked out the engine and connectors and such like, running 100%
OK.

I disconnected the battery for 2 hours, to clear any fault codes,
restarted, still have the same light, took the car to my mechanic, whose has a full test setup, first used a new diagnostic tester, would not "see" the ECU, which was weird, so he then put on a old diagnostic tester, which was used on the car about a year ago, which at the time worked fine, again did not "see" or could talk to the ECU.

Any idea of what it could be? not normal is it?

The car is running fine, OK, just a tiny, and I mean tiny ( could be me) smell of petrol when you start it up, could be me, though!

Slight tweak to subject line - Welcome to the backroom you should get some help - PU