MOT fail on emissions (Not by much)

Actually runs great, Obviously has been a problem for some time however since although it passed last years MOT this was only because (Somehow) it had been tested (and passed) as a pre-cat car??

Live data seems credable across the board but both lambdas are stuck at 4.9v and the live data also indicates running on open loop. Thermostat seems to open early and engine struggles to get past 83C

With oil cap removed (to generate weak running, confirmed by gas tester) ECU still reports 4.9 V for the lambdas, ECU even reports 4.9 volts with the lambdas disconnected?

What I havent done (and should have) was test the open circuit signal signal from the lambdas. but I will be going back to have another look tomorrow.

I am planning to replace the thermostat and see if that makes any diference (I am not holding my breath however, but I seem to remember reading that the BMW ECM wont enter closed loop below somewhere around 80C so there could be a borderline issue there)

Now, My questions are...

1) Can anybody confirm (or otherwise) the ECM stratagy regarding temperature?

2) Might there be an ECM issue regarding the fact that the live data still reports 4.9V with the lambdas disconnected! (IE Has anybody seen this one before)

3) Any other suggestion for tomorrows campaign of action??

Cheers

Paul