Cheers Steve,
Looking at the wiring diagram the lambda ground does not go directly to the ECU, but is earthed to the battery via a soldered joint. If I were to do this, I'd remove the existing earth and route a new one into the -ve on the battery. If the ECU earth is floating, then this needs to be sorted out too as without a decent earth, it won't have an adequate reference anyway.
The car passed it's MOT finally. After checking all connections to all sensors and re-making a pipe with a vacuum leak, I had the CO emissions down to 1%. So, happy with this, I changed the CAT and now am down to 0.1% at fast idle. Normal idle is at 0.48%, so scraped a pass there - but it's on the road again!
Thanks all, Mark