Well, after some more time looking at vagcom and not wanting to risk the ECU swap until there was no other choice I decided to swap over the first lambda sensors again and see what happened. The results from vagcom were pretty much as expected with the lambda values varying from 1.25 to 1.2998. A weak lambda combined with a rich tailpipe told me that the ECU just wasn't seeing reality so I decided to start the diagnosis from the beginning again, focussing on the first lambda sensor and using the Elsawin guidance.
The approach paid off. By starting at the beginning again and carefully carrying out each test I discovered than terminal 3 of the connector did not have the required 12-14 v. After carefully checking again and following teh wiring diagram, what I realised was that the connector was not wired up correctly, pins 1,3 and 5 were transposed with pins 2,4 and 6.
When I first got the car as a non runner only one of the 6 pins actually had a wire connection and one of the jobs i did while the engine was out was to rewire the lambda plug. I suppose my pathetic excuses would include working upside down with short cables with a connector I hadn't worked with before.
I finished rewiring by inspection lamp last night and got up early this morning to test all 5 connections to the ECU and the ignition feed. I fired the car up and within 5 minutes the first lambda had gone into closed loop mode and was regulating between 0.99 and 1.01 lambda
I have done some running around and the car is much more responsive, especially above 4000 rpm. Both the lambda sensors are passing tests, the lambda voltage is around 1.52 volts, however the catalyst test 046 is not passing with error 16804 : Bank 1: Efficiency Below Threshold. I also can't get the test for evap purge test on block 070 to initiate manually, but think it should pass the test again after a bit more driving around. I'm still stuck with my old friend error 17911
I have yet to take the car for an emissions test, but on my old Gunson CO meter the CO was down to 0% so that's a good sign
With the benefit of hindsight I don't understand why it took me such a long time to realise the very basic mistake I had made. I suppose you can call it experience.
Many thanks for all the help, patience and suggestions that helped me get there in the end