Well having spent the evening working through the figures various and based on the advice info above (gratefully received) I reckon that during the FY 22/23 I will be looking at a tax bill of just over £26K (based on current figures). Calculations are simplified by being on the AFPS 75 only, and I used the 16x change in value to get to the overall benefit for each year, allowing me to reveal the approximate available allowance from each of the previous 3 years (40,000 - (change x 16)) to offset the increase during the 'killer year'. Without the ability to offset, I would have been facing a bill more like £59K.
Of course, we have yet to see anything anything firmer than blancmange regarding McLeod; my understanding is that the sunset date for other schemes is Mar 22. I take this to mean that wef Apr 22, we will all be forced onto an inferior scheme (irrespective of age).