Just to update on this
Fitted the new Lambda. The previous slight surging that I had been experiencing went after this.
At the same time I changed the o-ring on the DISA valve becasue the old one had slight damage - although I don't think it was leaking.

It passed teh NCT retest today - so happy with that.:smiling:

Once I get a chance I'll plug in the diagnostics again to see how fast the new lambda swings - to answer my own question.