So just by way of update, I got a very quick reply from the SVPA. They confirm that it is the figure on your P60 that stipulates what your LTA use is. I think the lightbulb is coming on that my LTA is applied against what was a £1,800,000 limit in 2011/12 and does not get affected by any future changes to that limit (effectively frozen in time?) until and as GreenX states, I get another Crystallisation event (BCE). For a lot of ex Service persons, that will usually be a company pension during a second career. It looks like the LTA might get moved back to £1.8 Million again tomorrow, so my next BCE might well be under the same criteria.
Please do let me know if I have got any of this wrong