Hi all and thanks again for all the responses.

Well I had another 10 minutes to spare again today so took a different
secondary tapping to try to get more voltage headroom. Following that
tweak, I now have 25.5VDC available across the large electrolytics.
This is obviously plenty enough for 13.8VDC regulated; but is it now
*too* much to drop efficiently?
Thanks,
p.